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Distribution, Abundance, and Breeding Activities of the Least Bell's Vireo at Marine Corps Base Camp Pendleton, California—2021 Annual Report

Open-File Report 2023-1096
Ecosystems Mission Area—Species Management Research
Prepared in cooperation with Assistant Chief of Staff, Environmental Security, U.S. Marine Corps Base Camp Pendleton
By: , , and 
https://doi.org/10.3133/ofr20231096

Links

Acknowledgments

This work was funded by Environmental Security Department, Resources Management Division, Marine Corps Base Camp Pendleton, California. Data either are not available or have limited availability owing to restrictions of the funding entity (U.S. Marine Corps). Contact Ryan Besser, ryan.besser@usmc.mil, for more information. The authors thank the biologists who assisted in data collection for this project: Lisa Allen, Armand Amico, Annabelle Bernabe, Aaron Gallagher, Rachelle McLaughlin, Jessica Medina, Ben Stubbs, Devin Taylor, and Stéphane Vernhet.

Parts of this report were written following a previously developed template to maintain consistent presentation of results.

Executive Summary

The purpose of this report is to provide the Marine Corps with an annual summary of abundance, breeding activity, demography, and habitat use of endangered Least Bell’s Vireos (Vireo bellii pusillus) at Marine Corps Base Camp Pendleton (MCBCP or “Base”). Surveys for the Least Bell's Vireo were completed at MCBCP, California, between April 5 and July 13, 2021. Core survey areas and a subset of non-core areas in drainages containing riparian habitat suitable for vireos were surveyed three to four times. We detected 551 territorial male vireos and 26 transient vireos in core survey areas. An additional 98 territorial male vireos were detected in non-core survey areas. Transient vireos were detected on 8 of the 10 drainages/sites surveyed (core and non-core areas). Of the vireo territories in core areas, 89 percent were on the four most populated drainages, with the Santa Margarita River containing 70 percent of all territories in areas surveyed on Base. In core areas, 75 percent of male vireos were confirmed as paired; 76 percent of male vireos in non-core areas were confirmed as paired.

The number of documented Least Bell’s Vireo territories in core survey areas on MCBCP decreased 18 percent from 2020 to 2021. The number of territories in all but two core survey area drainages decreased by one or more between 2020 and 2021. The decrease in vireo numbers on MCBCP (18 percent) was consistent with population changes in surrounding areas, including the lower San Luis Rey River (24-percent decrease) and the middle San Luis Rey River (6-percent decrease).

Most core-area vireo territories (59 percent of males) were in willow (Salix spp.) riparian habitat. An additional 7 percent of birds occupied willow habitat co-dominated by Western sycamores (Platanus racemosa) or Fremont cottonwoods (Populus fremontii). Of all the territories surveyed, 25 percent were in riparian scrub dominated by mule fat (Baccharis salicifolia) or sandbar willow (S. exigua). Upland scrub was used by 8 percent of vireos; 1 percent of vireo territories were in non-native vegetation, and less than 1 percent of vireo territories were in alder or drier habitats co-dominated by coast live oak (Quercus agrifolia) and sycamore.

In 2019, MCBCP began operating an artificial seep along the Santa Margarita River; then, in 2021, two additional artificial seeps became operational. The artificial seeps pumped water to the surface starting in March and ending in August each year during daylight hours and were designed to increase the amount of surface water present to enhance Southwestern Willow Flycatcher (Empidonax traillii extimus) breeding habitat. Although this enhancement was designed to benefit flycatchers, few flycatchers have inhabited the seep areas within the past several years; therefore, vireos were selected as a surrogate species to determine effects of the habitat enhancement. This report presents the second year of analyses of vireo and vegetation response to the artificial seeps.

We sampled vegetation in two Seep sites and two Reference sites to determine the effects of a new water diversion dam that was completed in 2019 and two seep pumps that were installed to enhance surface water along the Santa Margarita River in 2019 and 2021. We measured higher total vegetation cover below 2 meters (m) at Seep sites than at Reference sites and lower total vegetation cover above 5 m at Seep sites than at Reference sites. Native herbaceous cover was also higher below 4 m at Seep sites than at Reference sites. Woody cover was lower above 5 m at Seep sites than at Reference sites. Soil moisture did not differ between Seep and Reference sites.

The U.S. Geological Survey has been color banding Least Bell’s Vireos on Marine Corps Base Camp Pendleton since 1995. In 2021, we continued to color band and resight color banded Least Bell’s Vireos to evaluate adult site fidelity, between-year movement, and the effect of surface-water enhancement on vireo site fidelity and between-year movement. We banded 164 Least Bell's Vireos for the first time during the 2021 season. Birds banded included 3 adult vireos and 161 juvenile vireos. All adult vireos were banded with unique color combinations. The juvenile vireos (all nestlings) were banded with a single gold numbered federal band on the right leg.

There were 52 Least Bell's Vireos banded before the 2021 breeding season that were resighted and identified on Base in 2021. Of these vireos, 45 were banded on Base, 6 were originally banded on the San Luis Rey River, and 1 was banded at Marine Corps Air Station, Camp Pendleton. Adult birds of known age ranged from 1 to at least 7 years old.

Base-wide survival of vireos was affected by sex, age, and year. Males had a slightly but significantly higher survival rate than females. Adults had a higher survival rate than first-year vireos. Survival of both adults and first-year birds was high from 2007 to 2008 and from 2012 to 2013 and low from 2020 to 2021. The return rate of adult vireos to Seep or Reference sites ranged from 45 to 57 percent.

Most returning adult vireos showed strong between-year site fidelity. Of the adults present in 2020 and 2021, 84 percent (94 percent of males; no females) returned to within 100 m of their previous territory. The average between-year movement for returning adult vireos was 0.1±0.2 kilometer (km). The average movement of first-year vireos detected in 2021 that fledged from a known nest on MCBCP in 2020 was 1.1±0.7 km.

We monitored Least Bell's Vireo pairs to evaluate the effects of surface-water enhancement on nest success and breeding productivity. Vireos were monitored at two Seep sites and two Reference sites. Early in 2021, a seep was installed in a 2020 Reference site, which changed the status of this monitoring site from Reference to Seep.

Nesting activity was monitored between April 5 and July 22 in 50 territories within the Seep and Reference sites (25 at Seep sites and 25 at Reference sites). All territories, except one, were occupied by pairs and all were fully monitored, meaning all nesting attempts were monitored at these territories. During the monitoring period, 97 nests (42 in Seep sites and 55 in Reference sites) were monitored.

Breeding productivity was similar at the Seep site and Reference sites (3.6 and 3.4 young per pair, respectively), with 84 percent of Seep pairs and 88 percent of Reference pairs successfully fledging at least one young in 2021. Seep sites had a higher proportion of all eggs that hatched and also a higher proportion of nests with eggs that hatched than Reference sites. Seep sites and References sites had similar proportions of hatchlings that fledged and nests with hatchlings that fledged. According to the best model, daily nest survival in 2021 was higher in Seep sites than in Reference sites. Completed nests at the Seep site were more likely to be successful than nests at Reference sites in 2021. At Seep sites, 75 percent of nests fledged young, whereas 53 percent of nests at Reference successfully fledged young. Vireos at Reference sites had to expend more energy in extra nest-building and egg-laying to produce a similar number of young as vireos at Seep sites. Predation was believed to be the primary source of nest failure at both sites. Predation accounted for 100 percent and 83 percent of nest failures at Seep and Reference sites, respectively. Failure of the remaining nests was attributed to infertile eggs and other unknown causes.

There were 11 plant species used as hosts for vireo nests in 2021. Successful vireo nests at Reference sites were further from the edge of host plants (closer to the center) and further from the edge of the nest plant clump than unsuccessful nests. Vireo nests at Seep sites were further from the edge of the host plant and the nest plant clump than vireo nests at Reference sites.

Introduction

The purpose of this report is to provide the Marine Corps with an annual summary of abundance, breeding activity, demography, and habitat use of endangered Least Bell’s Vireos (Vireo bellii pusillus) at Marine Corps Base Camp Pendleton (MCBCP or “Base”). The results are intended to provide the Base with biological information during each year to assist with appropriate management of the federally listed Least Bell’s Vireo and maintain compliant actions supporting military training on MCBCP in accordance with the Base Integrated Natural Resources Management Plan and U.S. Fish and Wildlife Service Programmatic Biological Opinion (U.S. Fish and Wildlife Service, 1995).

The Least Bell's Vireo (hereafter “vireo”) is a small, migratory songbird that breeds in southern California and northwestern Baja California, Mexico, from April through July. Historically abundant within lowland riparian ecosystems, vireo populations began declining in the late 1900s as a result of habitat loss and alteration associated with urbanization and conversion of land adjacent to rivers to agriculture (Franzreb, 1989; U.S. Fish and Wildlife Service, 1998; Riparian Habitat Joint Venture, 2004). Additional factors that contributed to the vireo's decline were (1) the expansion in range of the Brown-headed Cowbird (Molothrus ater), a brood parasite, to include the Pacific coast (U.S. Fish and Wildlife Service, 1986; Franzreb, 1989; Kus, 1998, 1999; Kus and others, 2020) and (2) the introduction of invasive non-native plant species, such as giant reed (Arundo donax), into riparian systems. By 1986, the vireo population in California was thought to number about 300 territorial males (U.S. Fish and Wildlife Service, 1986).

In response to the dramatic reduction in numbers of Least Bell's Vireos in California, the California Fish and Game Commission listed the species as endangered in 1980, and the U.S. Fish and Wildlife Service followed suit in 1986. Since listing, the vireo population in southern California has rebounded, largely in response to cowbird control and habitat restoration and preservation (Kus and Whitfield, 2005). As of 2006, the statewide vireo population was estimated to be approximately 2,500 territories (U.S. Fish and Wildlife Service, 2006), roughly a third of which occurred on MCBCP.

Male Least Bell's Vireos arrive on breeding grounds in southern California in mid-March. Male vireos are conspicuous and frequently sing their diagnostic primary song from exposed perches throughout the breeding season (Kus and others, 2020). Females arrive approximately 1–2 weeks after males and are more secretive. Females often are seen early in the season traveling through habitat with males. The female, with the male's help, builds an open cup nest in dense vegetation approximately 1 meter (m) above the ground. Clutch size for Least Bell's Vireos averages three to four eggs. Typically, the female and male incubate the eggs for 14 days, and young fledge from the nest at 11–12 days of age. It is not unusual for vireos to renest after a failed attempt, provided ample time remains within the breeding season (Kus and others, 2020). Vireos rarely fledge more than one brood in a season, although double-brooding can be more common during some years when breeding conditions are favorable (for example, early nest initiation, high early fledging success; Lynn and Kus, 2009, 2010a). Nesting lasts from early April through mid- to late July, but adults and juvenile birds remain on the breeding grounds into late September or early October before migrating to their wintering grounds in southern Baja California, Mexico.

Vireo pairs hold territories of approximately 0.5–1.0 hectares and maintain territory boundaries through vocal interactions with neighboring pairs. Territories remain stable throughout the breeding season, although silent males occasionally venture beyond their territory boundaries. Females sometimes leave their original territory to begin a new breeding attempt with a different male after completing an earlier nesting attempt (either successful or failed). Territory boundaries relax near the end of the breeding season as fledglings explore surrounding habitat. Territory fidelity between years is high for males, with typically 70–90 percent of males returning to within 100 m of their previous breeding territory (Rourke and Kus, 2006, 2007, 2008; Lynn and Kus, 2009, 2010a, 2010b, 2011, 2012, 2013; Lynn and others, 2014, 2015, 2016, 2017, 2018, 2020, 2024).

In 2019, MCBCP began operating an artificial seep along the Santa Margarita River as part of a Conjunctive Use Project. Two additional seeps were installed and activated in early 2021. The artificial seeps pump water to the surface from March through August each year during daylight hours and were designed to increase the amount of surface water present to enhance Southwestern Willow Flycatcher (Empidonax traillii extimus) breeding habitat. Although these enhancements were designed to benefit Southwestern Willow Flycatchers, few flycatchers have inhabited the seep areas within the past several years (Howell and Kus, 2015, 2016, 2017, 2024a, 2024b; Howell and others, 2018, 2020). However, Least Bell’s Vireos are abundant in the enhancement areas and were selected as a surrogate species to determine the effects of the habitat enhancement. Vireos frequently co-occur with flycatchers in riparian habitat and have similar habitat requirements, such as the presence of riparian obligate trees (typically willows and cottonwoods) with a brushy understory. Vireos and flycatchers have similar territory size and similar territorial behavior (singing from high perches to advertise territory boundaries), and they share some similarities in nest placement (nests are placed in the understory vegetation). Although there are some differences in habitat requirements between these two species (flycatchers prefer more mesic conditions that include surface water or elevated soil moisture during at least part of the breeding season; vireos are more tolerant of dryer, brushier vegetation sometimes lacking an overstory), vireos were abundant within the artificial seep areas, and their habits and requirements were considered sufficiently similar to flycatchers to serve as a surrogate species to evaluate the response of habitat to surface water enhancement and the effect of these habitat changes on vireo breeding productivity. This report presents preliminary analyses of vireo and vegetation response to the artificial seeps.

The purpose of this study was to document the status of the Least Bell's Vireo population on Marine Corps Base Camp Pendleton in San Diego County, California. Specifically, our goals were to (1) determine the size and composition of the breeding vireo population on Base; (2) characterize habitat used by vireos; (3) band a subset of vireos to facilitate the estimation of vireo annual survival and movement; (4) document the vegetation structure and plant composition within the areas affected by artificial seeps (Seep sites) compared to similar areas without artificial seeps (Reference sites); and (5) assess the effects of the artificial seeps on the vireo population by measuring annual survival, inter-annual movement, nest success, and breeding productivity of vireos in sites surrounding artificial seeps compared to Reference sites.

Data collected from this study are critical to inform natural resource managers about the status of this endangered species at MCBCP and guide modification of land use and management practices as appropriate to ensure the species’ continued existence.

This work was funded by and performed in cooperation with the Assistant Chief of Staff, Environmental Security Resources Management Division, Marine Corps Base Camp Pendleton, California. All activities were covered under 10(a)1(A) Recovery Permit #ESPER0004080-0.

Study Areas and Methods

Population Size and Distribution

Most of the MCBCP’s primary drainages, and several minor ones supporting riparian habitat (fig. 1), were surveyed for vireos between April 5 and July 13, 2021. Field work was completed by U.S. Geological Survey biologists Lisa Allen, Armand Amico, Annabelle Bernabe, Aaron Gallagher, Rachelle McLaughlin, Jessica Medina, Ben Stubbs, Devin Taylor, Michelle Treadwell, and Stéphane Vernhet.

In 2020, we began a new program for surveying for vireos on MCBCP. The new survey program involved surveying a core area plus a rotating subset of all non-core riparian habitat each year rather than surveying the entire Base every year. Selection criteria for surveys within the core area included (1) primary drainages (Santa Margarita River, Las Flores Creek, San Onofre Creek, and San Mateo Creek); (2) historic Southwestern Willow Flycatcher territories; (3) vireo nest monitoring areas from a previous post-fire study (Lynn and others, 2014, 2015, 2016, 2017, 2018, 2020); and (4) the survey unit with the highest average count of flycatchers from 2005 to 2014 in drainages where no historic flycatcher breeding or nest monitoring has occurred. Core survey areas were surveyed four times per year, at least 10 days apart, every year. Non-core areas were divided into five groups (A–E; fig. 1), each to be surveyed on a rotational schedule once every 5 years. Group C non-core areas were surveyed in 2021. The number of surveys per year in non-core areas varied depending on the amount of suitable habitat, the likelihood of vireo occurrence in the area, and logistical restrictions (for example, denial of range access). All non-core areas were surveyed three times in 2021, except the Stagecoach and Pump Road Monitoring Area sections of the Santa Margarita River and the Las Flores Creek non-core area, each of which was surveyed four times. The specific areas surveyed are shown in the “Core Areas” and “Rotating Non-Core Areas: Group C” sections.

1. Aerial view of Camp Pendleton with colored polygons depicting survey areas
Figure 1.

Least Bell's Vireo survey areas at Marine Corps Base Camp Pendleton, 2021.

Core Areas

  1. 1. De Luz Creek South, between the confluence of the Santa Margarita River and the confluence with Roblar Creek (appendix 1, fig. 1.1).

  2. 2. Santa Margarita River:

    1. (a) Air Station East, Effluent Seep, Bell North, Bell South: from Basilone Road to a point approximately 8.5 kilometers (km) downstream on the east side of the Santa Margarita River (appendix 1, figs. 1.1, 1.2).

    2. (b) Rifle Range, Pump Road (excluding Pump Road Monitoring Area): from the Rifle Range along Stagecoach Road to a point approximately 2.5 km downstream on the west side of the Santa Margarita River (appendix 1, figs. 1.1, 1.2).

    3. (c) Above Hospital, Below Hospital East, Below Hospital West: from the confluence with De Luz Creek to Basilone Road (appendix 1, fig. 1.1).

  3. 3. Lake O’Neill section of Fallbrook Creek, all riparian habitat surrounding Lake O’Neill (appendix 1, fig. 1.1).

  4. 4. Aliso Creek, between the Pacific Ocean and 0.5 km upstream of the electrical transmission lines (appendix 1, fig. 1.2).

  5. 5. Las Flores Creek (within Las Pulgas Canyon):

    1. (a) Lower Las Flores South: between the Pacific Ocean and a point approximately 2 km upstream from Stuart Mesa Road (appendix 1, fig. 1.3).

    2. (b) Upper Las Flores North: between a point 1.6 km downstream of Basilone Road to the Zulu Impact Area, approximately 0.75 km upstream from Basilone Road (appendix 1, fig. 1.3).

  6. 6. San Mateo Creek, Lower San Mateo Bottom: from the Pacific Ocean to a point 3.7 km upstream, including habitat south and east of the abandoned agricultural fields (appendix 1, fig. 1.4).

  7. 7. San Onofre Creek, Lower San Onofre East: from a point 1.5 km upstream from the Pacific Ocean to a point approximately 5 km upstream from the Pacific Ocean (appendix 1, fig. 1.4).

  8. 8. Pilgrim Creek, Pilgrim South: between the southern Base boundary and Vandegrift Boulevard, including the two side drainages east of Pilgrim Creek (appendix 1, fig. 1.6).

Rotating Non-Core Areas: Group C

  1. 1. French Creek, between the Pacific Ocean and the Edson Range Impact Area (appendix 1, fig. 1.2).

  2. 2. Las Flores Creek (within Las Pulgas Canyon), Lower Las Flores North: from 2 km upstream from Stuart Mesa Road to 3.8 km upstream from Stuart Mesa Road (appendix 1, fig. 1.3).

  3. 3. Santa Margarita River:

    1. (a) Ash Road: from the intersection of Vandegrift Boulevard and Ash Road upstream approximately 1 km ending on Carnes Road (appendix 1, fig. 1.2).

    2. (b) Stagecoach: all riparian habitat west of Stagecoach Road between the south end of Stagecoach Road and Basilone Road and all habitat east of Stagecoach Road and west of the Santa Margarita River from Basilone Road south approximately 0.7 km (appendix 1, fig. 1.1).

    3. (c) Pump Road Monitoring Area (south half of Pump Road): all riparian habitat west of the Santa Margarita River approximately 2.5 km to approximately 3.8 km south of Stagecoach Road (appendix 1; fig. 1.2).

  4. 4. Tuley Canyon, between the Base boundary and a point approximately 1.1 km upstream (appendix 1; fig. 1.2).

  5. 5. San Mateo Creek, Upper San Mateo: from a point approximately 1 km upstream from San Mateo Drive northeast to the Base boundary (appendix 1; figs. 1.4, 1.5).

Biologists followed standard survey techniques described in the U.S. Fish and Wildlife Service Least Bell's Vireo survey guidelines (U.S. Fish and Wildlife Service, unpub. data, 2001). Observers moved slowly (1–2 km per hour) through riparian habitat while searching and listening for vireos. Observers walked along the edge(s) of the riparian corridor on the upland or river side where habitat was narrow enough to detect a bird on the opposite edge. In wider stands, observers traversed the habitat to detect all birds throughout its extent. Surveys typically began at sunrise and were completed by early afternoon, avoiding conditions of high winds and extreme heat that can reduce bird activity and detectability.

All male Least Bell’s Vireos were detected and confirmed audibly by hearing their diagnostic song. Attempts were made to observe males visually to note banding status but direct observation was not required to confirm the identity of the species because the song was considered the most diagnostic field characteristic. The presence of a female vireo within a territory was confirmed audibly through the detection of the “pair call,” a unique call elicited between mated birds, or visually when observed traveling quietly with the male. Alternatively, female presence was inferred by observing a nest, the presence of dependent fledglings, or breeding behavior such as a food carry. For each bird detected, investigators recorded age (adult or juvenile), sex, breeding status (paired, unpaired, undetermined, or transient), and if the bird was banded. Birds were considered transients if they were not detected on two or more consecutive surveys after an initial detection. Vireo locations were mapped on 1:12,000 aerial photographs as well as 1:24,000 U.S. Geological Survey topographic maps, using Samsung Galaxy S7 and S8 and LG G5 mobile phones that use Android operating systems with a built-in Global Positioning System (GPS) to determine geographic coordinates (World Geodetic System of 1984 [WGS 84]).

Habitat Characteristics

Dominant native and non-native plants were recorded, and percent cover of non-native vegetation was estimated using cover categories of less than 5, 5–50, 51–95, and greater than 95 percent within the area used by each vireo detected. The overall habitat type within each territory was specified according to the following categories:

  • Mixed willow riparian: Habitat dominated by one or more willow species, including black willow (Salix gooddingii), arroyo willow (S. lasiolepis), and red willow (S. laevigata), with mule fat (Baccharis salicifolia) as a frequent co-dominant.

  • Willow-cottonwood: Willow riparian habitat in which Fremont cottonwood (Populus fremontii) is a co-dominant.

  • Willow-sycamore: Willow riparian habitat in which Western sycamore (Platanus racemosa) is a co-dominant.

  • Sycamore-oak: Woodlands in which sycamore and coast live oak (Quercus agrifolia) occur as co-dominants.

  • Riparian scrub: Dry or sandy habitat dominated by sandbar willow (S. exigua) or mule fat, with few other woody species.

  • Upland scrub: Coastal sage scrub adjacent to riparian habitat.

  • Non-native: Sites vegetated exclusively with non-native species, such as giant reed and salt cedar (Tamarix ramosissima).

Artificial Seep Study

In April 2019, MCBCP completed construction of a weir system designed to divert water from the Santa Margarita River to Lake O’Neill and several recharging ponds for the Conjunctive Use Project (Vernadero Group Inc., unpub. data, 2018). The purpose of the Conjunctive Use Project is to provide additional water for MCBCP and the Fallbrook Public Utility District (Vernadero Group Inc., unpub. data, 2018). In January 2019, MCBCP began operating an artificial seep along the Santa Margarita River to compensate for groundwater withdrawal upstream (fig. 2; U.S. Fish and Wildlife Service, 2016). Two more artificial seeps were installed in early 2021. A low-volume (20‒40 liters per minute), shallow groundwater irrigation pumping well was installed at each artificial seep location to draw water to the surface. The solar-powered pumps directed water to as many as six outlet pipes at each seep arranged within an area of approximately 1,500 square meters (m2). Water was pumped to the surface during daylight hours starting in March and ending in August each year. Shallow pools created by the seep pumps were small (8–44 m2) and limited to the immediate vicinity of the outlet pipes. The purpose of our study was to measure the effects of the artificial seeps on vegetation and vireo breeding, movements, and survival rates compared to areas where seeps were not operating, beginning in 2020, the first breeding season after the Conjunctive Use Project was implemented. Although the intent was to enhance historic Willow Flycatcher breeding habitat, few Willow Flycatchers have occupied these areas during the past 5 years (Howell and Kus, 2015, 2016, 2017, 2024a; Howell and others, 2018, 2020). Least Bell’s Vireos were selected as surrogates for Willow Flycatchers because they were well represented in the areas, and their habitat affinities are similar to those of Willow Flycatchers.

We established two types of study plots: Seep and Reference sites (fig. 2). Two Seep sites were selected to surround and extend downstream from (1) the seep installed in 2019 (northwest of the Old Treatment Ponds area) and (2) one of the seeps installed in 2021 (in the Pump Road area). A Reference site was selected 0.5–0.8 km from each Seep site. Reference sites were on the same side of the Santa Margarita River as their corresponding Seep sites and encompassed similar vegetation as the corresponding Seep site. We anticipate that the Reference sites will become drier relative to the Seep sites as the upstream water diversion effects are manifested.

Vegetation Structure and Plant Composition

We sampled vegetation at two Reference sites and two Seep sites (fig. 2) to examine the response of riparian habitat to locally augmented surface water. We collected vegetation data at 12 vireo territories per site (24 territories at Reference sites and 24 territories at Seep sites), centered on a single vireo nest per territory, for a total of 48 vegetation sampling locations (appendix 2). Vegetation data were collected using a protocol that combined aspects of flycatcher vegetation sampling in 2001 and 2002 (Rourke and others, 2004) and the “stacked cube” method developed to characterize canopy architecture in structurally diverse riparian habitat for vireos (Kus, 1998). Each sampling location consisted of a center plot (nest location) and 3 satellite plots (fig. 3), totaling 192 sampling plots. Satellite plots were located 15 m from the center plot, at 0, 120, and 240 degrees. We collected a GPS point at the center of each plot.

2. Topographic map of Santa Margarita River with colored polygons for nest monitoring sites and colored triangles for seep pump outlets.
Figure 2.

Location of Least Bell's Vireo Seep and Reference sites at Marine Corps Base Camp Pendleton, 2021.

3. Four circles, one at the center and one each at three equidistant compass points, diameter of circles and distance between centers of circles indicated.
Figure 3.

Vegetation sampling plot configuration. Abbreviation: m, meter.

Vegetation cover within 5 m of the center of the plot was visually estimated at seven height intervals: 0–1 m, greater than 1–2 m, greater than 2–3 m, greater than 3–4 m, greater than 4–5 m, greater than 5–6 m, and greater than 6 m. Overall foliage cover was recorded as the percentage of volume (percent cover) occupied by all foliage in the plot at each height interval, lumping all species together. Overall non-native foliage cover was measured as the percent cover of all non-native species (herbaceous and woody) within the plot at each height interval. Overall foliage and non-native cover were estimated using a modified Daubenmire (1959) scale with cover classes: less than 1, 1–10, 11–25, 26–50, 51–75, 76–90, and greater than 90 percent. Cover classes were further refined using “+” and “−” to indicate whether the estimate was in the upper or lower range of the cover class. A 7.5-m-tall fiberglass telescoping pole, demarcated in 1-m intervals, was used to determine height class and canopy height. We described the composition of vegetation at each height by recording the percentage of the overall foliage cover made up by each of the three species (Species 1, 2, and 3) contributing the most cover, as well as a fourth category of “All Other” species, with the four cover estimates summing to 100 percent. We also measured canopy height (estimated if greater than 7.5 m) and recorded soil moisture (percentage of relative saturation) at the center of each plot using a Kelway model HB-2 soil pH and moisture meter (kelinstruments.com/kelway-hb-2, Teaneck, New Jersey).

Vireo Survival, Site Fidelity, and Movement

We began color banding Least Bell’s Vireos on MCBCP in 1995. The primary goals of banding Least Bell's Vireos on MCBCP were to (1) evaluate adult and first-year annual survival; (2) evaluate vireo site fidelity within a potential source population; (3) investigate natal dispersal on Base and the role MCBCP young play in potentially supporting vireo populations off Base; and, starting in 2020, (4) evaluate how artificial seeps affected vireo site fidelity, dispersal, and survival. The regional Least Bell’s Vireo color banding convention designates orange or gold as the color representing MCBCP; therefore, nestlings from monitored nests were banded with a single anodized gold numbered federal band on the right leg at 6–7 days of age. When identification of neighboring territories was in question, adult vireos within Seep and Reference sites were captured in mist nets and banded with a unique combination of colored plastic and anodized metal bands, including either an anodized gold or orange plastic band (or both, depending on the available color combinations) to designate MCBCP as the bird’s site of origin. Returning adults previously banded as nestlings with a single numbered federal band were target netted to determine their identity, and their original band was supplemented with other bands to generate unique color combinations to enable identification of individuals.

Survival Estimates

During surveys and nest monitoring activities, we attempted to resight all vireos to determine if they were banded, and if so, to confirm their identity by reading their unique color band combination or by recapturing birds with single federal bands. We used resighting and recapture data from core survey areas and nest monitoring areas to calculate annual survival rates.

Annual survival was calculated for (1) adults Base-wide; (2) first-year vireos that were banded as nestlings or juveniles Base-wide (in other words, first-year survival); (3) adults that were initially detected at Seep or Reference sites; and (4) first-year vireos that were banded as nestlings or juveniles at Seep or Reference sites.

Imperfect detectability of banded individuals is typical of mark-recapture studies and occurs for various reasons (for example, females are more cryptic and may be missed on surveys, birds are detected as banded but their full color combinations [and thus identities] are not obtained; birds with single federal bands are not recaptured, and thus, their identities not determined). To account for individuals that were present but not detected, we used RMark (White and Burnham, 1999; Laake, 2013) to model survival rates and detection probability of vireos from 2005 to 2021 (see the “Annual Survival” section).

Site Fidelity and Movements

Site fidelity and movements of vireos were determined by measuring the distance between the center of a vireo’s breeding or natal territory in 2020 and the center of the same vireo’s breeding territory in 2021. Vireos demonstrated site fidelity if they returned to within 100 m of their 2020 territory (Kus and others, 2020).

Site fidelity and movement were calculated for the same four categories analyzed for annual survival (see earlier in text), except only individuals with known territory locations during the last year detected before 2021 were included (for example, juveniles banded after fledging were excluded because their natal territories could not be confirmed because of their capacity for substantial movement; vireos captured at either of the Monitoring Avian Productivity and Survivorship [MAPS] stations on Base were excluded unless their territory locations were known from surveys).

Nest Success and Breeding Productivity

Our purposes for monitoring Least Bell’s Vireo nests were to evaluate how vireo nest success and breeding productivity were affected by alteration of vireo habitat by the artificial seeps compared to reference sites with no augmented surface water. We monitored vireo nests at two Seep sites and two Reference sites to compare differences between the two groups. We monitored vireo nesting activity at 25 territories in Seep sites and 25 territories in Reference sites between April 5 and July 22, 2021. Territories were chosen in order of the vireos’ arrival. Vireos were observed for evidence of nesting, and their nests were located. Nests were visited as infrequently as possible to minimize the chances of leading predators or Brown-headed Cowbirds to nest sites; typically, there were three to five visits per nest. The first visit was timed to determine the number of eggs laid, the next few visits to determine hatching and age of young, and the last to band nestlings. Fledging was confirmed through detection of young outside the nest, or rarely, the presence of feather dust in the nest. Unsuccessful nests were placed into one of four nest fate categories: (1) “Depredated,” nests found empty or destroyed before the estimated fledge date and where the adult vireos were not found tending fledgling(s); (2) “Parasitized,” previously active nests that were subsequently abandoned by adult vireos after one or more Brown-headed Cowbird eggs were laid in the nest or any nests that fledged cowbird young without fledging vireo young; (3) “Other,” nests failing for reasons, such as poor nest construction, the collapse of a host plant that caused a nest’s contents to be dumped onto the ground, the presence of a clutch of infertile eggs, or other causes that were known; and (4) “Unknown,” nests that appeared intact and undisturbed but were abandoned with vireo eggs or nestlings. Characteristics of nests were recorded after abandonment or fledging of young from nests. These characteristics included nest height, host species, host height, and the distance nests were placed from the edge of the host plant and to the edge of the vegetation clump in which they were placed.

To determine if the artificial seeps affected vireo productivity, we compared vireo nest success (the percentage of vireo nests that successfully fledged at least one young) and breeding productivity (the number of eggs, nestlings, and fledglings produced). We examined nest success and the proportion of nests that were depredated or parasitized by cowbirds, and the likelihood of renesting after a first nesting attempt (successful or failed), to associate the effects altered habitat may have on the vulnerability of vireo nests to predators and brood parasites. We also examined clutch size (the maximum number of vireo eggs known to be laid in the nest), the proportion of eggs that hatched, the proportion of nestlings that fledged, the proportion of eggs that produced fledglings, the proportion of nests that successfully fledged young, the total number of fledglings per pair, and the proportion of pairs that had at least one successful nest. We examined vireo nest placement (nest height, height of the host plant, distance of the nest from the edge of the host plant, and distance of the nest from the edge of the vegetation clump that contained the host plant) to explore vireo response to potential differences in vegetation structure between Seep and Reference sites.

Marine Corps Base Camp Pendleton implements an intensive annual cowbird control program on Base, and parasitism of Least Bell’s Vireo nests is extremely rare. Nevertheless, when necessary, we followed our standard protocol for manipulating nest contents in the event cowbird eggs or nestlings were detected in vireo nests. In nests with fewer than three vireo eggs, cowbird eggs were removed no sooner than the seventh day of incubation to minimize the possibility of nest abandonment in response to the removal. Cowbird eggs were removed from nests containing three or more vireo eggs as they were found. Cowbird nestlings were removed immediately from nests.

Data Analyses

Population Size and Distribution

Because we began a core plus rotating non-core survey design in 2020, we limited our examination of annual differences to vireo territories that were located within the core survey areas. We present summaries of vireo territories in non-core survey areas without annual comparisons. Non-core survey results will be summarized every fifth year (for example, in 2024, 2029, and so on) when a round of non-core surveys has been completed. We examined annual differences in the dates vireos arrived and established breeding territories by compiling the total number of vireo territories established by the end of each month (April, May, June, and July) within core survey areas, all of which were surveyed at least four times annually for the past 17 years. We also calculated the projected Base-wide vireo population size in 2021 based on the proportion of all vireo territories that were counted within core survey areas from 2005 to 2019. First, for each year from 2005 to 2019, we calculated the proportion of all vireo territories on Base that were counted within the core survey areas (core/total); then, we multiplied these proportions by the number of vireo territories that were counted within core survey areas in 2021 to arrive at a range of projected numbers of vireo territories Base-wide in 2021.

Vegetation Structure and Plant Composition

At each height category, the estimates of the top three species contributing the most cover and the fourth category representing “All Other” were converted to percent cover values of the sampling plot area (n=192) by dividing the estimate by the overall foliage cover at that height. We then calculated average percent cover of each plant species, overall foliage cover, non-native plant species, canopy height, and soil moisture across each sampling location (center and three satellites) to obtain means for each territory (n=48). We further classified plant species into native herbaceous vegetation, woody vegetation (including both native and non-native species), and all herbaceous vegetation to calculate percent cover of each of these three groups at each height category and sampling location. We also identified the maximum canopy height at each sampling location. We used Student’s t-tests to determine if there were differences between Seep and Reference sites in (1) average canopy height, (2) maximum canopy height, (3) soil moisture, (4) vegetation volume (percent cover) of all plant species (overall foliage cover), (5) volume of non-native species (including herbaceous and woody species), (6) volume of native herbaceous species, and (7) volume of woody species at each height category by sampling location. We used Pearson’s correlation to examine the relationship between soil moisture at Seep site plots and the distance of the plot from the seep outlets at the sampling plot scale. We also used Pearson’s correlation to examine the relationship between soil moisture at both Seep and Reference sites combined and (1) canopy height, (2) percentage of overall foliage cover at each height category, and (3) percentage of herbaceous cover (including non-native herbaceous species) at all height categories at the sampling location scale.

Annual Survival

Base-Wide Survival

We analyzed annual survival of banded vireos on MCBCP in Program MARK (White and Burnham, 1999) using the RMark package (Laake, 2013) in R (R Core Team, 2022). Survival analysis in Program MARK accounts for individuals that were present but not captured (detected) by modeling both survival and recapture probabilities. Annual survival models were built for 2005–21 by creating an encounter history matrix of all individual vireos ever detected in MCBCP core survey areas, as well as the Pump Road Monitoring Area, and if they were observed in each year from 2005 to 2021. In the encounter history, a 1 is used if the bird was observed and a 0 if the bird was not observed. We included the Pump Road Monitoring Area because, although it is not a core survey area, we resight for banded birds there every year in the course of our demographic monitoring activities. Although nest monitoring sites were visited more frequently than core survey areas, we assumed detectability was the same between these two areas because we used broadcasted songs to enhance detectability of vireos. We rarely detected banded birds for the first time after the second survey, indicating that we were able to resight and identify almost all vireos by the end of May, regardless of their location.

Vireos were grouped by age (first-year [birds first encountered and banded as nestlings or juveniles] versus adult [birds first encountered and banded as adults and any first-year bird that survived to adulthood]) and sex (female versus male). Annual survival was assumed to be constant for adults once they survived their first year. We created two sets of models. In the first set, which included only adults (n=760), we modeled the effect of sex, year, and precipitation over the bio-year immediately preceding and partially encompassing the current breeding season (July 1–June 30; Office of Water Resources, 2021) on adult vireo survival and constrained detection probability to be affected by sex and year. For these models, we instructed MARK to use the encounter history containing all birds but excluded the first-year interval for any bird first encountered as a juvenile (in other words, we removed the juvenile to adult time interval). Detection probability was forced to account for sex because of sex-related behaviors (males are more obvious than females) and year because of annual differences in observers, number of surveys, and survey conditions (for example, surveys started late in 2011). We created six adults-only models: (1) the constant model (no covariates, describing annual survival when none of our covariates was allowed to account for variability); (2) sex (describing the effect of sex on survival); (3) precipitation (describing the effect of precipitation on survival); (4) year (describing annual differences in survival); (5) sex + precipitation (describing the additive effects of sex and precipitation); and (6) sex + year (describing the additive effects of year and sex). The second set of models included adults and first-year birds (n=2,613) and examined the effect of age, year, and bio-year precipitation on survival. We constrained detection probability to be affected by year to account for annual differences, as described in the first set of models. This model set did not include sex because we were unable to determine sex of vireos banded as nestlings unless they returned and were recaptured and identified as adults. Therefore, only the nestlings that survived their first winter could be classified retroactively as male or female, which severely biases the estimate of sex-related survival of first-year vireos. We created six age-related models: (1) the constant model (no covariates, describing annual survival when none of our covariates was allowed to account for variability); (2) age (describing the effect of age on survival); (3) precipitation (describing the effect of precipitation on survival); (4) year (describing annual trends in survival); (5) age + precipitation (describing the additive effects of precipitation and age group); and (6) age + year (describing the additive effects of year and age group). Survival estimates were derived from the top model. Models created for survival in RMark only included detections from sites at which survey effort has been consistent from 2005 to 2021 (including MCBCP core survey areas, artificial seep study nest monitoring areas, the lower San Luis Rey River, and the middle San Luis Rey River). Incidental resights outside of these survey sites were excluded from analysis. Additionally, we did not include detections from MAPS captures because MAPS effort was considered different from survey effort. We excluded 17 adults with unknown sex from our first model set analysis because we were not interested in defining characteristics of this group.

We also calculated annual survival for Seep and Reference sites between 2020 and 2021. Because there was only a single year interval, we did not use RMark for this analysis but instead presented rough redetection rates for adults and first-year vireos (apparent survival). Calculations for adults were limited to birds that were detected within Seep or Reference sites in both 2020 and 2021. Calculations for first-year birds included all nestlings from successful nests that were banded in 2020 and were redetected anywhere in 2021.

Model Evaluation

We used an information-theoretic approach (Akaike’s Information Criterion for small sample sizes [AICc]; Burnham and Anderson, 2002) to evaluate support for models regarding the effects of sex, age, year, and precipitation on vireo survival. For the adults-only model set, we hypothesized that females would have lower survival rates than males. We used logistic regression with a logit link to build and rank six models by AICc (the model with the lowest AICc was the highest ranked model). Models were considered well supported if they were within 2 AICc of the highest-ranked (top) model (ΔAICc [difference in AICc] less than 2). We further examined the top models by calculating the odds ratio for each covariate in the model (the odds that the covariate had an effect on survival such that no effect equaled 1, negative effect was less than 1, positive effect was greater than 1) and then examining the 95-percent confidence interval of the odds ratio. For example, if the 95-percent confidence interval of the odds ratio was greater than 1 (and did not include 1), we had 95-percent confidence that the covariate had a positive effect on survival relative to the reference.

For the model set that included first-year vireos, we hypothesized that first-year survival would be lower than adult survival. We used logistic regression with a logit link and built six models with combinations of age, year, and bio-year precipitation before ranking these models from lowest to highest AICc. Because we were only interested in the effects on first-year survival rates, we eliminated models where the effect was only on survival beyond the first year.

Nest Success and Breeding Productivity

We used chi-square or Fisher’s Exact tests to determine if there were differences between Seep and Reference sites in (1) the likelihood of vireos renesting after a first nesting attempt, (2) the likelihood of renesting if the first nesting attempt failed or was successful, (3) the proportion of nests that successfully fledged young, (4) the proportion of nests that were depredated, (5) whether or not the first nest attempt was successful, (6) the proportion of eggs that hatched, (7) the proportion of nestlings that fledged, (8) the proportion of eggs that produced fledglings, (9) the proportion of nests that produced fledglings, and (10) the number of pairs that had at least one successful nest. Chi-square tests were used when sample sizes were sufficient; Fisher’s Exact tests were used when one or more categories contained fewer than five samples.

We used Student’s t-tests to determine if there were differences between Seep and Reference sites in the (1) number of nesting attempts, (2) clutch size, and (3) number of fledglings per pair. If nests were parasitized by Brown-headed Cowbirds, rescued by removing the cowbird egg(s) or nestling(s), and subsequently fledged vireo young, all success and productivity calculations were rerun treating successful rescued nests as failed nests to estimate the potential effect(s) of cowbird parasitism on the Pendleton vireo population.

Data were analyzed using Program R (R Core Team, 2022). Two-tailed tests were considered significant if P≤0.10. Means are presented with standard deviations. All data from the MCBCP from 2005 to 2020 used in comparisons with current data can be found in Rourke and Kus, 2006, 2007, 2008; Lynn and Kus, 2009, 2010a, 2010b, 2011, 2012, 2013; Lynn and others, 2014, 2015, 2016, 2017, 2018, 2020, 2024. Data from before 2005 were extracted from unpublished reports by Griffith Wildlife Biology (unpub. data, 2004).

Daily Nest Survival

We used RMark (White and Burnham, 1999; Laake, 2013) in program MARK to calculate daily survival rate (DSR) of vireo nests, which accounts for the variability in exposure days across nests found at different stages of the nesting cycle and allows for the analysis of the effects of covariates on DSR (Dinsmore and others, 2002). Using RMark, we modelled the effects of the Seeps on DSR. Nest survival was calculated across a 32-day cycle length: 2 days for the last day of nest construction and a day of rest before the first egg is laid, 4 days for egg-laying, 14 days for incubation, and 12 days for the nestling period. Age of nests at the time they were discovered was calculated in days by forward- or backward-dating of nests in relation to known dates of nest-building, egg-laying, or hatching. Data compiled for each nest included (1) the Julian dates for when the nest was first found, last active, and last checked; (2) the nest fate (successful or unsuccessful); (3) the age of the nest when it was initiated, relative to the first nest found that year; (4) if the nest was in a Seep site or a Reference site; and (5) the year. RMark uses program MARK to create models with or without covariates (user-designated) and produces metrics for evaluating the validity of each model or how well the model fits the data relative to the other models. We used AICc (see the “Annual Survival” section) to evaluate support for nest survival models reflecting a priori hypotheses regarding the effect of Seeps on DSR. We hypothesized that DSR would be higher in Seep sites than in Reference sites and that DSR would decrease annually in Reference sites as the habitat became drier relative to the Seep sites. We used logistic regression with a logit link to build models. First, we generated a constant survival model to serve as a reference for the effect of treatment on DSR. We then created models with treatment, year, the combined effect of treatment and year (treatment + year), and the interaction of treatment and year (treatment * year) and evaluated support for the models in relation to the constant survival model. We examined the well supported models further by calculating the odds ratio for each covariate in the model (the odds that the covariate had an effect on survival such that no effect equaled 1, negative effect was less than 1, or positive effect was greater than 1) and then examining the 95-percent confidence interval of the odds ratio.

Nest Characteristics

We used Student’s t-tests to determine if there were differences between Seep and References sites in (1) nest height, (2) host plant height, (3) diameter of the host plant canopy at nest height, (4) distance to the edge of the host plant, and (5) distance to the edge of the vegetation clump in which the nest was located. We also used Student’s t-tests to determine if there were differences in nest placement characteristics between successful and failed nests within Seep and Reference sites.

Results

Population Size and Distribution

Core Survey Areas

A total of 577 male Least Bell’s Vireos were detected in core survey areas during Base-wide surveys (table 1; fig. 4; appendix 3, figs. 3.1–3.13). Of these vireos, 551 were territorial males, 74 percent of which were confirmed as paired, and 26 were transients. This total represents an 18-percent decrease in territorial males from the same areas surveyed in 2020 (669). Transient vireos were observed on all 8 (100 percent) of the drainages and sites surveyed. Most vireo territories (89 percent) were on the four most populated drainages/sites (Santa Margarita River, Las Flores Creek, San Mateo Creek, and San Onofre Creek), and 69 percent of vireo territories were along the Santa Margarita River, which is the largest expanse of riparian vegetation on Base (tables 1, 2). The remaining 4 drainages and sites each contained 20 or fewer territories.

4. Vertical bars indicating vireo territory numbers, bar height decreasing and increasing by year.
Figure 4.

Number of Least Bell’s Vireo territories in core survey areas at Marine Corps Base Camp Pendleton, 2005–21.

Table 1.    

Number and distribution of Least Bell’s Vireos in core survey areas at Marine Corps Base Camp Pendleton, 2021.

[ha, hectare]
Drainage/survey site Territories Total
territories		 Transients Total area
surveyed
(ha)
Known
pairs		 Single/status
undetermined
Santa Margarita River, Interstate-5 to De Luz Creek 294 86 380 13 964
De Luz Creek South 13 4 17 2 95
Lake O'Neill section of Fallbrook Creek 7 5 12 2 98
Aliso Creek 8 6 14 2 94
Las Flores Creek—Pacific Ocean to Stuart Mesa Road 5 3 8 0 124
Las Flores Creek—Stuart Mesa Road to eastern edge of lower core area 17 8 25 1 138
Las Flores Creek—Western edge of upper core area to Zulu Impact Area 10 9 19 0 83
San Onofre Creek, lower east core area 13 10 23 3 191
San Mateo Creek, lower bottom core area 30 5 35 1 492
Pilgrim Creek, Base boundary upstream to Vandegrift Boulevard 13 5 18 2 78
Total 410 141 551 26 2,359
Table 1.    Number and distribution of Least Bell’s Vireos in core survey areas at Marine Corps Base Camp Pendleton, 2021.

From 2005 to 2019, 55–62 percent of resident males detected on MCBCP were within the core areas (average 57±2 percent). Assuming the vireo population on the rest of MCBCP in 2021 did not vary from the 2005 to 2019 distribution, we estimated between 886 and 1,010 resident vireos on MCBCP in 2021.

The distribution of Least Bell’s Vireo territories documented on Base in 2021 remained similar to 2020 across all core survey areas (table 2). From 2020 to 2021, the percentage of vireo territories in each drainage (the number of vireo territories detected in the drainage divided by the total number of vireo territories in core survey areas) changed by less than 1 percent in every drainage except at San Mateo Creek (increased by 1 percent) and Pilgrim Creek (decreased by 2 percent). The absolute number of vireo territories decreased in every drainage except San Onofre Creek. The Santa Margarita River, although continuing to support the most vireo territories, decreased by 17 percent (80 territories). Las Flores Creek, the second most populated drainage, decreased by 19 percent (12 territories). Pilgrim Creek decreased by 45 percent (15 territories), De Luz Creek decreased by 26 percent (6 territories), Aliso Creek decreased by 18 percent (3 territories), and Fallbrook Creek (Lake O’Neill section) decreased by 20 percent (3 territories). San Mateo Creek remained the same as in 2020, and San Onofre Creek increased 5 percent (one territory).

Table 2.    

Number of territorial male Least Bell’s Vireos in core survey areas at Marine Corps Base Camp Pendleton, by drainage, 2005–21.

[Number includes only singing males determined to hold territories. Numeric change is the positive or negative change in the number of vireo territories between 2020 and 2021]
Number of territorial males
Drainage 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021 Numeric
change
Santa Margarita River 314 276 282 326 402 435 293 255 292 296 261 281 254 367 333 460 380 −80
De Luz Creek 11 19 17 19 24 23 17 19 21 15 12 12 7 9 16 23 17 −6
Fallbrook Creek 14 5 7 10 8 10 5 4 5 7 3 3 5 13 8 15 12 −3
Aliso Creek 21 11 9 11 21 16 9 8 9 6 4 6 5 9 9 17 14 −3
Las Flores Creek 51 43 46 41 59 64 47 28 33 38 31 29 24 47 48 64 52 −12
San Onofre Creek 13 10 11 7 17 13 14 16 16 12 9 10 16 11 10 22 23 1
San Mateo Creek 28 22 21 29 48 43 29 22 26 23 29 35 25 31 35 35 35 0
Pilgrim Creek 28 16 17 16 15 18 20 12 19 16 16 13 15 21 22 33 18 −15
Total 480 402 410 459 594 622 434 364 421 413 365 389 351 508 481 669 551 −118
Table 2.    Number of territorial male Least Bell’s Vireos in core survey areas at Marine Corps Base Camp Pendleton, by drainage, 2005–21.

Least Bell’s Vireos began arriving on Base during the last week of March 2021, with 62 percent of territories established by the end of April (fig. 5). This number represents the second lowest percent of territories established by the end of April since 2005. By the end of May, 88 percent of territories had been established. The first vireo detected on MCBCP in 2021 was found on March 31, which was 7 days later than the median documented first arrival date for vireos (March 24) since 2005. Note that first detection dates represent anecdotal observations; standardized vireo surveys began on April 5 in 2021, but vireo presence before surveys was noted when observed.

5. Vertical bars for each year divided into four sections for months, section sizes fluctuate.
Figure 5.

Percentage of all Least Bell’s Vireo territories established by the end of each month in core survey areas at Marine Corps Base Camp Pendleton, 2005–21. Surveys began late in 2011 and 2012; therefore, arrival dates for these years are not included.

Non-Core Survey Areas

A total of 103 male vireos were detected in non-core survey areas in 2021 (table 3); 82 percent of territorial males were confirmed as paired, and three transients were detected.

Table 3.    

Number and distribution of Least Bell’s Vireos in non-core survey areas at Marine Corps Base Camp Pendleton, 2021.

[ha, hectare]
Drainage/survey site Territories Total
territories		 Transients Total area
surveyed
(ha)
Single/status
undetermined		 Known
pairs
Santa Margarita River, Interstate-5 to De Luz Creek 68 6 74 2 189
French Creek 2 5 7 0 35
Las Flores Creek 10 6 16 0 82
San Mateo Creek 0 0 0 1 284
Tuley Canyon 0 1 1 0 8
Total 80 18 98 3 599
Table 3.    Number and distribution of Least Bell’s Vireos in non-core survey areas at Marine Corps Base Camp Pendleton, 2021.

Habitat Characteristics

Core Survey Areas

Vireos used several different habitat types ranging from willow-dominated thickets along stream courses to non-native vegetation and upland scrub (table 4). Most vireo locations in core survey areas were in habitat characterized as mixed willow riparian, with 59 percent of males in the study area found in this habitat. An additional 7 percent of birds occupied willow habitat co-dominated by cottonwoods or sycamores. Twenty-five percent were found in riparian scrub, dominated by mule fat or sandbar willow, 8 percent of vireos occupied upland scrub, 1 percent occupied non-native habitat, and less than 1 percent were in alder or drier habitats characterized by a mix of sycamore and oak.

Table 4.    

Habitat types used by Least Bell’s Vireos in core survey areas at Marine Corps Base Camp Pendleton, 2021.

[Habitat types are included for resident and transient Least Bell’s Vireo locations. Abbreviations: >, greater than; <, less than]
Habitat type Number of locations Percentage
of total
>50 percent
native		 >50 percent
non-native		 Total
Mixed willow 319 24 343 59
Riparian scrub 132 15 147 25
Upland scrub 32 13 45 8
Willow-sycamore 32 0 32 6
Non-native 1 3 4 1
Willow-cottonwood 4 0 4 1
Sycamore-oak 1 0 1 <1
Alder 1 0 1 <1
Total 522 55 577 100
Table 4.    Habitat types used by Least Bell’s Vireos in core survey areas at Marine Corps Base Camp Pendleton, 2021.

The proportion of vireos documented in non-native vegetation in core survey areas decreased from 2020 to 2021 (table 5). In 2021, 10 percent (55/577) of vireos were in areas where non-native species comprised at least 50 percent of the habitat. Most of the territories dominated by non-native vegetation (62 percent) contained predominantly poison hemlock (Conium maculatum), and 27 percent contained predominantly black mustard (Brassica nigra). Five of the eight drainages in 2021 contained territories dominated by non-native vegetation. Two of these drainages (the Santa Margarita River and Las Flores Creek) also contained territories dominated by non-native vegetation every year since 2015. Overall, 2005 remained the year with the highest number of territories dominated or co-dominated by non-native vegetation.

Table 5.    

Proportion of all Least Bell’s Vireo territories dominated or co-dominated by non-native vegetation, by drainage, 2005–21. [—Left]
Drainage Proportion of all territories (number of territories within the drainage)
2005 2006 2007 2008 2009 2010 2011 2012
Aliso Creek 0.09 (23) 0.00 (14) 0.09 (11) 0.00 (12) 0.00 (23) 0.11 (18) 0.00 (9) 0.18 (11)
De Luz Creek 0.08 (12) 0.05 (20) 0.00 (17) 0.00 (21) 0.00 (22) 0.00 (23) 0.00 (17) 0.00 (20)
Fallbrook Creek 0.16 (19) 0.00 (6) 0.14 (7) 0.00 (10) 0.00 (8) 0.00 (10) 0.00 (5) 0.17 (6)
Las Flores Creek 0.00 (49) 0.07 (45) 0.00 (47) 0.24 (45) 0.09 (65) 0.12 (67) 0.04 (48) 0.00 (28)
Pilgrim Creek 0.00 (28) 0.00 (16) 0.00 (18) 0.00 (17) 0.30 (20) 0.05 (20) 0.05 (22) 0.00 (12)
San Mateo Creek 0.62 (26) 0.16 (25) 0.00 (19) 0.11 (27) 0.10 (52) 0.27 (49) 0.03 (30) 0.00 (21)
San Onofre Creek 0.45 (11) 0.00 (12) 0.00 (12) 0.43 (7) 0.30 (20) 0.13 (16) 0.25 (16) 0.00 (16)
Santa Margarita River 0.19 (319) 0.07 (291) 0.03 (291) 0.03 (328) 0.07 (422) 0.05 (439) 0.11 (297) 0.07 (258)
Total 0.18 (487) 0.06 (429) 0.03 (422) 0.06 (467) 0.08 (632) 0.07 (642) 0.09 (444) 0.06 (372)
Table 5.    Proportion of all Least Bell’s Vireo territories dominated or co-dominated by non-native vegetation, by drainage, 2005–21. [—Left]

Table 5.    

Proportion of all Least Bell’s Vireo territories dominated or co-dominated by non-native vegetation, by drainage, 2005–21. [—Right]
Drainage Proportion of all territories (number of territories within the drainage)
2013 2014 2015 2016 2017 2018 2019 2020 2021
Aliso Creek 0.00 (12) 0.00 (6) 0.00 (5) 0.00 (6) 0.20 (5) 0.00 (9) 0.08 (12) 0.11 (18) 0.13 (16)
De Luz Creek 0.00 (21) 0.25 (16) 0.19 (16) 0.19 (16) 0.71 (7) 0.00 (10) 0.13 (16) 0.43 (23) 0.00 (19)
Fallbrook Creek 0.00 (6) 0.00 (7) 0.00 (5) 0.00 (4) 0.00 (6) 0.13 (15) 0.13 (8) 0.27 (15) 0.07 (14)
Las Flores Creek 0.00 (34) 0.00 (39) 0.03 (33) 0.10 (30) 0.04 (23) 0.15 (55) 0.24 (49) 0.07 (67) 0.11 (53)
Pilgrim Creek 0.00 (19) 0.00 (18) 0.00 (17) 0.00 (13) 0.06 (16) 0.05 (21) 0.00 (22) 0.09 (33) 0.00 (20)
San Mateo Creek 0.00 (28) 0.00 (28) 0.17 (30) 0.00 (39) 0.40 (25) 0.00 (33) 0.29 (35) 0.23 (35) 0.00 (36)
San Onofre Creek 0.00 (16) 0.06 (16) 0.44 (9) 0.00 (11) 0.19 (16) 0.00 (11) 0.20 (10) 0.13 (24) 0.38 (26)
Santa Margarita River 0.05 (300) 0.04 (308) 0.08 (280) 0.03 (292) 0.04 (268) 0.06 (376) 0.13 (342) 0.18 (470) 0.10 (393)
Total 0.04 (436) 0.04 (438) 0.09 (395) 0.04 (411) 0.09 (366) 0.06 (530) 0.15 (494) 0.17 (685) 0.10 (577)
Table 5.    Proportion of all Least Bell’s Vireo territories dominated or co-dominated by non-native vegetation, by drainage, 2005–21. [—Right]

Non-Core Survey Areas

Most vireo locations in non-core survey areas (80 percent) were in habitat characterized as mixed willow riparian (table 6). The second most represented habitat type was riparian scrub (17 percent of territories), followed by upland scrub (2 percent of territories) and oak-sycamore habitat (1 percent of territories).

Table 6.    

Habitat types used by Least Bell’s Vireos in non-core survey areas at Marine Corps Base Camp Pendleton, 2021.

[Habitat types are included for resident and transient Least Bell’s Vireo locations. Abbreviation: >, greater than]
Habitat type Number of locations Percentage
of total
>50 percent
native		 >50 percent
non-native		 Total
Mixed willow 68 13 81 80
Riparian scrub 17 0 17 17
Upland scrub 1 1 2 2
Sycamore-oak 1 0 1 1
Total 87 14 101 100
Table 6.    Habitat types used by Least Bell’s Vireos in non-core survey areas at Marine Corps Base Camp Pendleton, 2021.

Vegetation at Artificial Seep and Reference Sites

Overall foliage cover was significantly greater at Seep sites than at Reference sites below 2 m (fig. 6; table 7). Conversely, overall foliage cover was significantly greater at Reference sites than at Seep sites above 5 m. Percent cover of native herbaceous species was greater at Seep sites than at Reference sites from 0 to 4 m. Non-native cover at Reference sites was greater between 4 and 6 m than at Seep sites. Percent cover of woody vegetation was significantly greater above 5 m at Reference sites than at Seep sites. Average canopy height was greater at Reference sites (8.0 m) than at Seep sites (6.3 m; t=−3.6, P<0.01), as was maximum canopy height (Reference: 11.7 m; Seep: 9.3 m; t=−2.6, P=0.01).

6. Horizontal bars colored by foliage type, stacked by height category. Length of bars shows percent foliage cover.
Figure 6.

Average total percent cover by height class and plant type at Seep and Reference sites, Santa Margarita River, Marine Corps Base Camp Pendleton, 2021. Error bars represent 1 standard deviation. Asterisk (*) indicates a significant difference (P≤0.10). Abbreviations: >, greater than; m, meter; %, percent; ≤, less than or equal to.

Table 7.    

Results of Student’s t-tests for differences in vegetation cover between the Seep site and Reference sites at Marine Corps Base Camp Pendleton, 2021.

[m, meter; t, Student’s t statistic; P, probability that the difference was not significant; >, greater than; —, no data]
Height
interval
(m)		 Overall
foliage cover		 Native
herbaceous cover		 Non-native
vegetation cover		 Woody
vegetation cover
t P t P t P t P
0–1 2.3 0.03 2.7 0.01 −1.4 0.16 1.1 0.28
>1–2 1.7 0.09 2.1 0.04 −1.3 0.19 1.0 0.33
>2–3 0.5 0.62 2.1 0.04 −0.8 0.45 0.0 0.99
>3–4 −0.4 0.66 2.0 0.05 −1.1 0.28 −0.7 0.52
>4–5 −1.3 0.19 −0.9 0.39 −1.7 10.10 −1.3 0.21
>5–6 −1.9 0.06 −1.0 0.33 −1.8 10.08 −2.0 10.05
>6 −2.8 0.01 — — −1.7 0.11 −2.9 10.01
Table 7.    Results of Student’s t-tests for differences in vegetation cover between the Seep site and Reference sites at Marine Corps Base Camp Pendleton, 2021.
1

Indicates statistically significant findings.

There was no difference in soil moisture between Seep (61±24 percent) and Reference sites (55±28 percent; t=−0.83, P=0.41, n=48), and soil moisture was not significantly correlated with canopy height (r=0.14, P=0.34, n=48). Within the Seep sites, soil moisture was not correlated with the distance from the nearest seep outlet at the sample plot level (r=−0.10, P=0.36, n=96). There was also no correlation between soil moisture and overall foliage cover at any height category or between soil moisture and herbaceous cover at any height category (table 8).

Table 8.    

Results of Pearson’s correlation tests for percentage of soil moisture versus overall foliage cover and percentage of soil moisture versus percentage of herbaceous cover at each height category, Marine Corps Base Camp Pendleton, 2021.

[m, meter; vs., versus; r, Pearson's correlation coefficient; P, probability that the difference was not significant; n, number of nests in sample (successful, unsuccessful, and in lowest tier: Seep, Reference); >, greater than; —, no data]
Height
interval
(m)		 Percentage soil moisture
vs.
overall foliage cover		 Percentage soil moisture
vs.
percentage herbaceous cover
r P n r P n
0–1 0.11 0.47 48 0.22 0.13 48
>1–2 0.00 0.99 48 0.10 0.49 48
>2–3 −0.09 0.53 48 0.14 0.34 48
>3–4 0.04 0.81 48 0.11 0.45 48
>4–5 −0.01 0.95 48 −0.01 0.95 48
>5–6 −0.02 0.92 48 −0.06 0.70 48
>6 0.02 0.89 48 — — —
Table 8.    Results of Pearson’s correlation tests for percentage of soil moisture versus overall foliage cover and percentage of soil moisture versus percentage of herbaceous cover at each height category, Marine Corps Base Camp Pendleton, 2021.

Soil moisture varied geographically across all four monitoring sites. The western edge of the Seep site in the Old Treatment Ponds area was dry except for one plot, despite being close to the main river channel (fig. 7). The highest soil moisture in the Old Treatment Ponds Seep site was recorded in scattered plots with little discernable pattern. Soil moisture was high on the eastern edge of the Old Treatment Ponds Reference site and the western side of the Pump Road Reference site. A dirt road defined the western border of the Pump Road sites; the middle section of this road was flooded at the beginning of the breeding season and remained wet (standing surface water) through late June. Soil was 100 percent saturated in at least one sampling plot at all but one sampling location at the Pump Road Seep site. Except for a small part of the northwest edge of the Pump Road Reference site, the soil type across all monitoring sites was uniformly high-draining riverwash or Greenfield sandy loam (U.S. Department of Agriculture Natural Resources Conservation Science, 2020).

7. Aerial view of Santa Margarita River: colored polygons demarcate monitoring sites, and colored dots show soil moisture at plots scattered throughout polygons.
Figure 7.

Percentage of soil moisture at vegetation sampling plots, Marine Corps Base Camp Pendleton, 2021.

Vireo Survival, Site Fidelity, and Movements

Returning Banded Birds

We were able to observe 1,027 adult Least Bell’s Vireos (671 males, 99 percent of all males, and 356 females, 73 percent of all females) on Base well enough to determine banding status in 2021, although not all banded vireos were observed well enough to conclusively identify the individual. At least 77 vireos had been banded before the 2021 breeding season, 6 of which we could not identify because the vireos were banded with only a single numbered gold or blue metal federal band as nestlings and not recaptured (5 “natals”) or had incomplete resights and were therefore not identified (1; table 9). There were 19 other vireos that were resighted with a single silver metal federal band and therefore could not be identified. Birds with a single silver band were either banded at local MAPS stations within the past 3 years or were nestlings banded at Marine Corps Air Station, Camp Pendleton (MCAS) in 2018–21 (Ferree and Clark, 2018, 2019, 2020, 2021). These 19 birds were not recaptured to identify the individual because they were not part of our focal group for this analysis. In total, we were able to identify 52 vireos that had unique color band combinations on Base in 2021 (table 9; appendix 4). Of the 52 identified banded vireos, 45 vireos had been banded on Base and 7 vireos were originally banded off Base (6 on the San Luis Rey River and 1 at MCAS, B. Kus, U.S. Geological Survey, unpub. data, 2015, 2018, 2019, 2021; Houston and others, 2021; table 10). Adult birds of known age ranged from 1 to at least 7 years old. At least 19 percent of adult banded birds were 1 year old in 2021.

Table 9.    

Banding status of Least Bell’s Vireos detected on Marine Corps Base Camp Pendleton (MCBCP) and those that emigrated off Base in 2021.

[Birds detected on MCBCP include immigrants. Natal vireos were originally banded as nestlings with a single numbered federal band. Abbreviation: —, no data]
Banding status Detected on MCBCP Total on
MCBCP		 Emigrants Total
Male Female Male
Uniquely banded before 2021 36 5 41 — 41
Natal recaptured in 2021 8 3 11 11 12
Subtotal of known identity vireos 44 8 52 1 53
Natal, not recaptured 1 4 5 — 5
Incomplete resight 0 1 1 — 1
Silver metal federal band 10 9 19 — 19
Grand total 55 22 77 1 78
Table 9.    Banding status of Least Bell’s Vireos detected on Marine Corps Base Camp Pendleton (MCBCP) and those that emigrated off Base in 2021.
1

Found on the San Luis Rey River in 2021.

Five natal vireos (one male and four females) were resighted on MCBCP in 2021 (table 9). Based on the color of the metal leg bands, three were banded as nestlings on Base before 2021 or at Marine Corps Air Station, Camp Pendleton before 2016, and two were banded as nestlings on the San Luis Rey River before 2021. Efforts to recapture and identify these vireos were unsuccessful.

One male vireo that was last seen when it was banded as a nestling on MCBCP in 2016 was detected off Base in 2021 (table 9). This male was observed on the lower San Luis Rey River, approximately 1 km west of Benet Road, in 2021.

Table 10.    

Number of banded adult Least Bell’s Vireos by original year banded, age, original banding location, and sex at Marine Corps Base Camp Pendleton in 2021.

[≥, greater than or equal to; yr(s), year(s)]
Year
originally
banded		 Age in
2021		 Number of vireos observed by origin
Marine Corps Base
Camp Pendleton		 San Luis Rey
River		 Marine Corps Air Station,
Camp Pendleton		 Unknown Total
Male Female Male Female Male Male Female
2015 >7 yrs 2 0 0 0 0 0 0 2
6 yrs 3 1 0 0 1 0 0 5
2016 5 yrs 1 0 0 0 0 0 0 1
2017 >5 yrs 3 0 0 0 0 0 0 3
4 yrs 2 0 0 0 0 0 0 2
2018 ≥4 yrs 0 1 0 0 0 0 0 1
3 yrs 0 0 1 0 0 0 0 1
2019 ≥3 yrs 3 1 0 0 0 0 0 4
3 yrs 2 0 0 0 0 0 0 2
2 yrs 6 1 3 1 0 0 0 11
2020 >2 yrs 13 2 0 0 0 0 0 15
1 yr 3 1 1 0 0 0 0 5
Subtotal 38 7 5 1 1 0 0 52
Unknown >1 yr 11 23 0 32 0 410 49 25
Total 39 10 5 3 1 10 9 77
Table 10.    Number of banded adult Least Bell’s Vireos by original year banded, age, original banding location, and sex at Marine Corps Base Camp Pendleton in 2021.
1

Vireo seen with a metal gold numbered band, indicating that it was originally banded at Marine Corps Base Camp Pendleton before 2021 or Marine Corps Air Station, Camp Pendleton before 2016.

2

Two vireos seen with a metal gold numbered band, indicating that they were originally banded at Marine Corps Base Camp Pendleton before 2021 or Marine Corps Air Station, Camp Pendleton before 2016. One vireo with incomplete resights of the bands. Identity and origin unknown.

3

Vireos seen with a metal dark blue numbered band, indicating that they were originally banded on the San Luis Rey River.

4

Vireos seen with a metal silver numbered band and assumed to be adult vireos banded by the San Diego Natural History Museum at Marine Corps Air Station, Camp Pendleton in 2018–20 (Ferree and Clark, 2018, 2019, 2020, 2021) or individuals banded at nearby Monitoring Avian Productivity and Survivorship stations in 2020 or 2021.

Newly Banded Birds

A total of 164 Least Bell's Vireos were captured and banded for the first time during 2021 (table 11). These newly banded birds included 3 adult vireos that were caught for the first time and banded with a unique color combination and 161 juvenile birds that were banded as nestlings with a single gold numbered federal band. These newly banded vireos are not included in site fidelity or movement analyses.

Table 11.    

Summary of new Least Bell’s Vireos captured and banded on Marine Corps Base Camp Pendleton in 2021.

[—, no data]
Age
banded		 Males Unknown
sex		 Total
Adult 3 — 3
Nestling — 161 161
Total 3 161 164
Table 11.    Summary of new Least Bell’s Vireos captured and banded on Marine Corps Base Camp Pendleton in 2021.

Survival, Site Fidelity, and Movement

Adult Survival

The most important variable for predicting adult survival was sex. Of the six models we created to describe factors affecting adult survival, the two top-ranked models contained sex (table 12), and in both models, males had slightly but significantly higher survival rates than females (table 13; average male survival=62±2 percent, average female survival=54±2 percent). The top-ranked model also contained bio-year precipitation, although the effect of precipitation on survival was weak and possibly did not significantly contribute to the model (95-percent confidence interval included 1). Both of the top-ranked models had more support than the constant model. The two highest-ranked models were well supported with AICc weights greater than 0.10 and ΔAICc less than 2. Year was only included in the two lowest-ranked models and was not as well supported as the constant model or precipitation-only model.

Table 12.    

Logistic regression models for the effect of sex (male versus female), year, and bio-year (July 1–June 30) precipitation on survival of adult Least Bell’s Vireos on Marine Corps Base Camp Pendleton, 2005–21.

[The effect of sex and year on detection probability was included in all models. Models are ranked from best to worst based on Akaike’s Information Criterion for small samples (AICc), the difference in AICc between this model and the top model (ΔAICc), and AICc weights. AICc is based on −2×loge likelihood and the number of parameters in the model. Abbreviation: +, plus]
Model AICc ΔAICc AICc
weight		 Number of
parameters		 Deviance
Sex+precipitation 2,504.1 0.0 0.41 20 502.6
Sex 2,504.4 0.3 0.35 19 505.0
Constant 2,507.3 3.2 0.08 18 509.9
Precipitation 2,507.3 3.2 0.08 19 507.9
Sex+year 2,507.9 3.8 0.06 34 477.4
Year 2,511.3 7.2 0.01 33 482.9
Table 12.    Logistic regression models for the effect of sex (male versus female), year, and bio-year (July 1–June 30) precipitation on survival of adult Least Bell’s Vireos on Marine Corps Base Camp Pendleton, 2005–21.

Table 13.    

Parameter estimate (β), standard error (SE), odds ratios, and 95-percent confidence intervals (CI) for top two models explaining annual survival of adult Least Bell’s Vireos on Marine Corps Base Camp Pendleton, 2005–21.

[Models are in order of best-supported to least-supported. Reference represents females. Abbreviation: +, plus]
Effect β SE Odds
ratio		 95-percent
CI
Reference 0.36 0.20 1.43 0.97–2.12
Male1 0.36 0.15 1.43 11.06–1.94
Precipitation −0.01 0.00 0.99 20.98–1.00
Reference 0.15 0.14 −0.13 0.88–1.53
Male1 0.35 0.15 0.05 11.05–1.92
Table 13.    Parameter estimate (β), standard error (SE), odds ratios, and 95-percent confidence intervals (CI) for top two models explaining annual survival of adult Least Bell’s Vireos on Marine Corps Base Camp Pendleton, 2005–21.
1

The 95-percent confidence interval of the odds ratio does not span 1, indicating that the effect is a significant contributor to the model.

2

Upper CI value rounded down to 1.00.

Adult and First-Year Vireo Survival

Of the six models we created to examine the effects of age, year, and precipitation on vireo survival, the model that included age and year outperformed all other models (table 14). The age + year model had an AICc weight greater than 0.99, which is well above any other model in the model set. In this model, age and number of years were significant sources of variation (95-percent confidence intervals did not include 1; table 15).

Adult vireo annual survival averaged 63±12 percent (range 33–77 percent), with highest survival from 2007 to 2008 and 2012 to 2013 and lowest survival from 2020 to 2021 (table 16). First-year vireo annual survival averaged 17±6 percent (range 5–27 percent) and showed the same trend across years as adult survival. Adult survival rates consistently exceeded first-year survival rates across all years.

Table 14.    

Logistic regression models for the effect of age (first-year versus adult), year, and bio-year (July 1–June 30) precipitation on survival of Least Bell’s Vireos on Marine Corps Base Camp Pendleton, 2005–21.

[The effect of sex and year on detection probability was included in all models. Models are ranked from best to worst based on Akaike’s Information Criterion for small samples (AICc), the difference in AICc between this model and the top model (ΔAICc), and AICc weights. AICc is based on −2×loge likelihood and the number of parameters in the model. Abbreviation: +, plus]
Model AICc ΔAICc AICc
weight		 Number of
parameters		 Deviance
Age+year 4,808.6 0.0 1.00 33 853.0
Age 4,831.8 23.2 0.00 18 906.7
Age+precipitation 4,833.2 24.6 0.00 19 906.1
Year 5,379.8 571.2 0.00 32 1,426.3
Constant 5,386.6 578.0 0.00 17 1,463.5
Precipitation 5,386.6 578.1 0.00 18 1,461.5
Table 14.    Logistic regression models for the effect of age (first-year versus adult), year, and bio-year (July 1–June 30) precipitation on survival of Least Bell’s Vireos on Marine Corps Base Camp Pendleton, 2005–21.

Table 15.    

Parameter estimates (β), standard errors (SE), odds ratios, and 95-percent confidence intervals (CI) for the top model explaining annual survival of Least Bell’s Vireos on Marine Corps Base Camp Pendleton, 2005–21.

[Reference represents first-year vireos, 2005–06. All other effects values are the difference between that parameter and the reference. Abbreviation: +, plus]
Effect β SE Odds
ratio		 95-percent
CI
Reference1 −1.90 0.34 0.15 10.08–0.29
Adults1 2.23 0.10 9.34 17.72–11.30
2006–07 0.65 0.45 1.91 0.79–4.62
2007–081 0.88 0.42 2.40 11.06–5.42
2008–09 0.37 0.39 1.45 0.68–3.11
2009–10 0.06 0.40 1.07 0.48–2.35
2010–11 −0.57 0.41 0.56 0.25–1.26
2011–12 0.38 0.46 1.46 0.59–3.61
2012–131 0.89 0.40 2.43 11.10–5.36
2013–14 0.45 0.40 1.56 0.72–3.40
2014–15 −0.15 0.39 0.86 0.40–1.84
2015–16 0.40 0.40 1.49 0.68–3.27
2016–17 −0.16 0.39 0.85 0.40–1.81
2017–18 0.58 0.38 1.79 0.85–3.80
2018–19 0.60 0.41 1.81 0.81–4.05
2019–20 −0.08 0.40 0.93 0.42–2.02
2020–211 −1.04 0.39 0.35 10.17–0.75
Table 15.    Parameter estimates (β), standard errors (SE), odds ratios, and 95-percent confidence intervals (CI) for the top model explaining annual survival of Least Bell’s Vireos on Marine Corps Base Camp Pendleton, 2005–21.
1

The 95-percent confidence interval of the odds ratio does not span 1, indicating that the effect is a significant contributor to the model.

Table 16.    

Annual survival rates for adult and first-year Least Bell’s Vireos on Marine Corps Base Camp Pendleton, 2005–21.

[Estimates were calculated from the top model. Abbreviations: ±, plus or minus]
Survival
interval		 Adult
survival
(percent)		 First-year
survival
(percent)
2005–06 58 13
2006–07 73 22
2007–08 77 26
2008–09 67 18
2009–10 60 14
2010–11 44 8
2011–12 67 18
2012–13 77 27
2013–14 69 19
2014–15 55 11
2015–16 68 18
2016–17 54 11
2017–18 72 21
2018–19 72 21
2019–20 56 12
2020–21 33 5
Mean±SD 63±12 17±6
Table 16.    Annual survival rates for adult and first-year Least Bell’s Vireos on Marine Corps Base Camp Pendleton, 2005–21.

Vireo Survival at Seep and Reference Sites

Of 11 adult vireos detected at Seep sites in 2020, 5 returned to Seep sites in 2021 (5/8 or 63 percent of males; and 0/3 or no females), which is an adult return rate of 45 percent to Seep sites. Of 14 adults identified at Reference sites in 2020, 8 returned to Reference sites in 2021 (7/11 or 64 percent of males and 1/3 or 33 percent of females) and, which is an adult return rate of 57 percent to Reference sites. Of the 11 adults identified at the 2020 Reference site that became a Seep site in 2021, 5 returned to Seep sites in 2021 (3/7 or 43 percent of males and 2/4 or 50 percent of females), which is an adult return rate of 45 percent.

In 2020, 107 banded nestlings fledged from Seep or Reference site nests. Of the 26 fledglings from the Seep site, 1 was redetected (4 percent) in 2021. Of 56 fledglings from Reference sites, 3 were redetected (5 percent) in 2021. None of the 25 fledglings from the 2020 Reference site that became a Seep site in 2021 were redetected in 2021.

Base-Wide Site Fidelity and Movement

Resighting banded birds allowed us to identify individuals that either returned to the same site they used in a previous year (within 100 m) or moved to a different location (appendix 5). There were 37 adult vireos (33 males and 4 females) identified at MCBCP in 2020 that were resighted in 2021, all of which occupied known territories both years. Most returning adult vireos showed strong between-year site fidelity. Of the 37 returning territorial adults, 31 (84 percent of territorial adults; 31 males, 94 percent of males; no females) occupied a breeding site in 2021 that they had defended in 2020 (within 100 m). Four additional vireos (11 percent of all vireos; two males, 6 percent of males; two females, 50 percent of females) returned to sites adjacent to their previous territories (within 300 m). The average distance moved by returning adult vireos was 0.1±0.2 km (range 0.0–1.1 km; 0.0±0.0 km, range 0.0–0.1 km for males; 0.4±0.4 km, range 0.1–1.1 km for females).

Of the 107 nestlings banded in 2020 on MCBCP, four were resighted in 2021 and occupied known territories (three males and one female; table 17). The average distance that first-year vireos moved from their natal territories to their breeding territories was 1.1±0.7 km (range 0.5–1.9 km; males moved 0.9±0.7 km, range 0.5–1.7 km; the female moved 1.9 km). One first-year male vireo that was banded as a nestling on the San Luis Rey River in 2020 was detected 4.7 km from his natal territory on MCBCP in 2021.

Table 17.    

Between-year dispersal into, within, or out of Marine Corps Base Camp Pendleton by Least Bell’s Vireos banded as juveniles in 2020 and detected in 2021.

[Drainage Codes: SMR, Santa Margarita River; SLR, San Luis Rey River; PL, Pilgrim Creek. Sex: M, male; F, female. Abbreviation: km, kilometer]
Drainage/territory Dispersal
distance
(km)		 Sex
2020 2021
SMR/TET SMR/PR11 0.5 M
SMR/KTM SMR/CAN 0.6 M
SMR/KYL SMR/ES30 1.7 M
SMR/KTM SMR/GIM 1.9 F
SLR1/CSCH PL/PS11 4.7 M
Table 17.    Between-year dispersal into, within, or out of Marine Corps Base Camp Pendleton by Least Bell’s Vireos banded as juveniles in 2020 and detected in 2021.
1

Immigrant to MCBCP from the San Luis Rey River.

Site Fidelity and Movement at Seep and Reference Sites

Adult fidelity to Seep and Reference sites was high. Of vireos detected in 2020 and 2021, all five of the vireos that held territories at the Seep site in 2020 returned to the Seep site in 2021 (100 percent). All eight of the vireos that held territories at Reference sites in 2020 returned to Reference sites in 2021 (100 percent; appendix 5). Four of the five vireos that held territories at the 2020 Reference site that became a Seep site in 2021 returned to the new Seep site (80 percent); the fifth vireo was a female that returned to the 2020 Seep site in 2021. Three first-year vireos that fledged from Reference sites in 2020 were redetected in 2021: one at a Reference site, one at the 2020 Reference site that became a Seep site in 2021, and one outside of the monitoring sites. One first-year vireo that fledged from a Seep site in 2020 returned to an area outside of our monitoring sites in 2021.

Nest Success and Breeding Productivity

Nesting activity was monitored at 25 territories in Seep sites and 25 territories in Reference sites (table 18; figs. 8–11; appendix 6). All of the territories were considered fully monitored, meaning that all nests within the territory were found and documented during the breeding season. One territory at a Reference site was occupied by a single male who began building a nest and then disappeared within 1 week. This territory was not included in productivity analyses. The remaining 49 pairs built 96 nests; 7 of these were not completed (“INC” or “FAL” in appendix 6) and have been excluded from calculations of nest success and productivity. Nests were not found for one pair at a Seep site.

Table 18.    

Number of Least Bell’s Vireo territories and nests monitored at Seep and Reference sites on Marine Corps Base Camp Pendleton, 2021.

[±, plus or minus]
Territories/nests Nest monitoring
area type
Seep Reference
Territories 125 25
Nests (number complete) 42 (40) 255 (49)
Completed nests per pair 1.6±0.6 2.0±1.0
Total number of nests per pair
(includes incomplete nests)		 1.7±0.7 2.3±1.1
Table 18.    Number of Least Bell’s Vireo territories and nests monitored at Seep and Reference sites on Marine Corps Base Camp Pendleton, 2021.
1

One pair did not build any nests.

2

Includes one false nest started and abandoned by a single male.

8. Aerial view of monitoring site, colored dots indicate vireo territories and seep outlets.
Figure 8.

Locations of monitored Least Bell’s Vireo territories at the Old Treatment Ponds Reference site, Marine Corps Base Camp Pendleton, 2021.

9. Aerial view of monitoring site, colored dots indicate vireo territories and seep outlets.
Figure 9.

Locations of monitored Least Bell’s Vireo territories at the Pump Road Reference site, Marine Corps Base Camp Pendleton, 2021.

10. Aerial view of monitoring site, colored dots indicate vireo territories and seep outlets.
Figure 10.

Locations of monitored Least Bell’s Vireo territories at the Old Treatment Pond Seep site, Marine Corps Base Camp Pendleton, 2021.

11. Aerial view of monitoring site, colored dots indicate vireo territories and seep outlets.
Figure 11.

Locations of monitored Least Bell’s Vireo territories at the Pump Road Seep site 2021, treated as a Reference site in 2020, Marine Corps Base Camp Pendleton, 2021.

Nesting Attempts

Pairs at Reference sites built more nests (including incomplete nests) than pairs at Seeps sites during the 2021 breeding season (table 18; t=−2.2, P=0.03). Pairs at Seep sites (16/24; 67 percent) and Reference sites (18/24; 75 percent) were equally as likely to renest after an initial nesting attempt (chi-square=0.10, P=0.75). The number of renests after a failed first nesting attempt did not differ between Seep pairs (8/9; 89 percent) and Reference pairs (12/12; 100 percent; Fisher’s Exact P=0.43). Similarly, pairs at Seep sites (8/15; 53 percent) and Reference sites (6/12; 50 percent) were equally likely to renest after a successful first nesting attempt (chi-square=0; P=1.00). Pairs at Reference sites were more likely to renest after a failed first nesting attempt than after a successful first nesting attempt in 2021 (Fisher’s Exact P=0.01), although pairs at Seep sites were equally likely to renest after a failed or successful first nesting attempt (Fisher’s Exact P>0.18). When both monitoring site types were combined, pairs were more likely to renest after a failed nesting attempt than they were after a successful nesting attempt in 2021 (Fisher’s Exact P=0.001). Overall, 95 percent (20/21) of vireo pairs attempted to renest after a failed first nesting attempt, and 52 percent (14/27) of pairs attempted to renest after a successful first nesting attempt in 2021. We did not find any nesting attempts for one pair at the Seep sites. Two pairs at Seep sites and five pairs at Reference sites attempted to nest three times, two pairs at Reference sites attempted to nest four times, and one pair at a Reference site attempted to nest five times in 2021.

Nest Success

Completed nests in the Seep site were more likely to be successful than completed nests in Reference sites (chi-square=3.65, P=0.06). At Seep sites, 75 percent (30/40) of nests successfully fledged young, whereas 53 percent (26/49) of those in Reference sites successfully fledged young (table 19). First nesting attempts were as likely to be successful at Seep sites (63 percent) as at Reference sites (50 percent; chi-square=0.34, P=0.53) in 2021 (appendix 6). Overall, 63 percent of all nesting attempts were successful, and 56 percent of first nesting attempts were successful in 2021.

Table 19.    

Fate of completed Least Bell’s Vireo nests in fully monitored territories at Seep and Reference sites, Marine Corps Base Camp Pendleton, 2021.

[Numbers in parentheses are proportions of total nests]
Nest fate Number of nests
Seep Reference Total
Successful 30 26 56 (0.63)
Predation 10 19 29 (0.33)
Parasitism 0 0 0 (0.00)
Other/unknown 0 4 4 (0.04)
Total completed nests 40 49 89 (1.00)
Table 19.    Fate of completed Least Bell’s Vireo nests in fully monitored territories at Seep and Reference sites, Marine Corps Base Camp Pendleton, 2021.

Causes of failure were similar at Seep and Reference sites. Most nest failures at Seep and Reference sites were caused by predation, although predation events were not witnessed (table 19). Predation accounted for 100 percent (10/10) of nest failures at the Seep sites and 83 percent (19/23) of nest failures at Reference sites. We documented four nests that failed for other reasons, known and unknown, at our monitoring sites (appendix 6). Three nests were abandoned with no eggs ever confirmed, and one nest contained eggs that never developed and were likely infertile. Overall, 25 percent and 47 percent of completed vireo nests at Seep and Reference sites were lost to predation or other causes, respectively.

Cowbird Parasitism

No Least Bell’s Vireo nests were parasitized by Brown-headed Cowbirds in 2021.

Productivity

Clutch size did not differ between Seep sites and Reference sites (table 20). However, the Seep site had a higher percentage of eggs that hatched and a higher percentage of nests with hatchlings than Reference sites. The percentage of hatchlings that fledged and the percentage of nests with hatchlings that ultimately fledged young did not differ between Seep and Reference sites. However, the proportion of eggs that produced fledglings was higher at Seep sites than at Reference sites. Seep and Reference pairs fledged similar numbers of young and were equally likely to fledge young. Nine pairs at Seep sites (36 percent) and five pairs at Reference sites (21 percent) successfully fledged two broods in 2021 (appendix 6). Vireo pairs at Seep and Reference sites combined fledged 3.5 vireo young per pair, and 86 percent of monitored pairs were successful in fledging at least one young in 2021.

Table 20.    

Reproductive success and productivity of nesting Least Bell’s Vireos at Seep and Reference sites, Marine Corps Base Camp Pendleton, 2021.

[±, plus or minus; %, percent; ≥, greater than or equal to]
Parameter Seep
sites		 Reference
sites		 Total
Nests with eggs 40 46 86
Eggs laid 136 153 289
Average clutch size1 3.5±0.6 3.6±0.6 3.5±0.6
Hatchlings 121 106 227
Nests with hatchlings 40 34 74
Eggs2 89% 69% 79%
Nests3 100% 74% 86%
Fledglings 89 81 170
Nests with fledglings 30 26 56
Hatchlings4 74% 76% 75%
Nests5 75% 76% 76%
Fledglings per egg6 30.7 30.5 0.6
Average number of young
fledged per pair7		 3.6±2.3 3.4±2.2 3.5±2.3
Pairs fledging≥1 young8 21 (84%) 21 (88%) 42 (86%)
Table 20.    Reproductive success and productivity of nesting Least Bell’s Vireos at Seep and Reference sites, Marine Corps Base Camp Pendleton, 2021.
1

Based on 36 Seep and 34 Reference non-parasitized nests with a full clutch (t=0.8; P=0.40).

2

Percentage of all eggs that hatched (chi-squared=15.4, P<0.001).

3

Percentage of all nests with eggs in which at least one egg hatched (Fisher’s Exact P<0.001).

4

Percentage of all hatchlings that fledged (chi-squared=0.1, P=0.73).

5

Percentage of all nests with hatchlings in which at least one young fledged (Fisher’s Exact P=1.00).

6

Proportion of all eggs that fledged (chi-squared=4.1, P=0.04).

7

Based on 25 Seep and 24 Reference pairs (t=−0.3, P=0.77).

8

Based on 25 Seep and 24 Reference pairs (Fisher’s Exact P=1.00).

Daily Nest Survival

Using treatment (Seep versus Reference site) and year as co-variates, we built five models with potential to predict the probability that a nest would survive from 1 day to the next (table 21). The constant model was generated first and was then superseded by higher-ranked models that contained treatment and year (see “Daily Nest Survival” section in the “Methods” section for details). The highest-ranked model contained only treatment and, in this model, DSR was significantly higher at Seep sites than at Reference sites (95-percent confidence interval of the odds ratio for treatment was greater than 1; table 22). Adding year and the interaction of year and treatment produced models that performed better than the constant model, but in both of these models, the 95-percent confidence intervals of the odds ratios for year, in addition to or interacting with treatment, all contained 1, and therefore, likely did not contribute significantly to the model. The constant model and the model that contained only year were greater than 2 ΔAICc from the highest-ranked model.

Table 21.    

Logistic regression models for the effect of year and treatment (if a nest was in a Seep or Reference site) on nest survival of Least Bell’s Vireos on Marine Corps Base Camp Pendleton, 2021.

[Models are ranked from best to worst based on Akaike’s Information Criterion for small samples (AICc), the difference in AICc between this model and the top model (ΔAICc), and AICc weights. Abbreviations: *, interacting with; +, plus]
Model AICc ΔAICc AICc
weight		 Number of
parameters		 Deviance
Treatment 409.7 0.0 0.42 2 405.7
Year*treatment 411.4 1.7 0.18 4 403.4
Year+treatment 411.4 1.7 0.18 3 405.4
Constant 411.9 2.2 0.14 1 409.9
Year 413.1 3.4 0.08 2 409.1
Table 21.    Logistic regression models for the effect of year and treatment (if a nest was in a Seep or Reference site) on nest survival of Least Bell’s Vireos on Marine Corps Base Camp Pendleton, 2021.

Table 22.    

Parameter estimate (β), standard error (SE), odds ratios, and 95-percent confidence intervals (CI) for models explaining daily survival rate of Least Bell’s Vireos at Seep and Reference sites across years on Marine Corps Base Camp Pendleton, 2021.

[Models are in order of best-supported to least-supported. Abbreviations: *, interacting with; +, plus]
Effect β SE Odds
ratio		 95-percent
CI
Reference1 3.77 0.15 43.33 132.53–57.71
Treatment (Seep)1 0.53 0.27 1.71 11.01–2.89
Reference1 3.81 0.20 45.38 130.51–67.50
Year (2021) −0.10 0.29 0.91 0.51–1.61
Treatment (Seep) 0.10 0.38 1.10 0.52–2.31
Year (2021)*treatment (Seep) 0.77 0.54 2.16 0.75–6.20
Reference1 3.71 0.18 40.96 128.74–58.39
Year (2021) 0.13 0.25 1.14 0.70–1.85
Treatment (Seep) 0.51 0.27 1.66 0.97–2.84
Table 22.    Parameter estimate (β), standard error (SE), odds ratios, and 95-percent confidence intervals (CI) for models explaining daily survival rate of Least Bell’s Vireos at Seep and Reference sites across years on Marine Corps Base Camp Pendleton, 2021.
1

The 95-percent confidence interval of the odds ratio does not span 1, indicating that the effect is a significant contributor to the model.

Nest Characteristics

Least Bell’s Vireos used 11 plant species for nesting at Seep and Reference sites in 2021, although not all plant species were used within each treatment (table 23). Vireos used nine species at the Seep site and nine species at Reference sites. Vireos placed 57 percent of all nests (55 percent at the Seep site and 58 percent at Reference sites) in arroyo willow, sandbar willow, or black willow. At the Seep site, all (42) vireo nests were placed in woody vegetation or perennial vines. At Reference sites, 50 vireo nests (91 percent) were placed in woody vegetation, and 5 (9 percent) were placed in herbaceous vegetation. All five vireo nests that were placed in herbaceous vegetation at the Reference site were built in non-native plant species (all in poison hemlock).

Table 23.    

Host plant species used by Least Bell’s Vireos at Seep and Reference sites, Marine Corps Base Camp Pendleton, 2021.

[Numbers in parentheses are proportions of total nests within treatment types. Abbreviation: —, no data]
Host species Number of nests
Seep Reference Total
Arroyo or red willow 15 (0.36) 25 (0.45) 40 (0.41)
Mule fat 7 (0.17) 6 (0.11) 13 (0.13)
Sandbar willow 6 (0.14) 6 (0.11) 12 (0.12)
Blue elderberry (Sambucus mexicanus) 2 (0.05) 7 (0.13) 9 (0.09)
Poison hemlock — 5 (0.09) 5 (0.05)
Poison oak (Toxicodendron diversilobum) 4 (0.10) 1 (0.02) 5 (0.05)
Coyote brush (Baccharis pilularis) 1 (0.02) 2 (0.04) 3 (0.03)
Salt cedar 1 (0.02) 2 (0.04) 3 (0.03)
Wild grape (Vitus sp.) 2 (0.05) — 2 (0.02)
California sycamore 2 (0.05) — 2 (0.02)
Dead willow 2 (0.05) — 2 (0.02)
Black willow — 1 (0.02) 1 (0.01)
Table 23.    Host plant species used by Least Bell’s Vireos at Seep and Reference sites, Marine Corps Base Camp Pendleton, 2021.

In 2021, we did not find differences in nest-site characteristics between successful and failed nests at Seep sites (table 24). At Reference sites, we found that successful nests were further from the edge of the host plant (closer to the center) and also further from the edge of the nest clump than unsuccessful nests. Vireo nests at Seep sites were further from the edge of the host plant and the nest clump than vireo nests at Reference sites (table 25).

Table 24.    

Least Bell’s Vireo nest characteristics and results of Student’s t-tests of successful versus unsuccessful nesting attempts at Seep and Reference sites, Marine Corps Base Camp Pendleton, 2021.

[n, number of nests in sample (successful, unsuccessful); t, Student’s t statistic; P, probability that the difference was not significant; m, meter]
Nest characteristic
(m)		 Nest fate n t P
Successful Unsuccessful
Average nest height 1.0 0.9 30, 11 −0.1 0.95
Average host height 4.5 5.5 30, 12 1.1 0.26
Average distance to edge of host 0.8 0.9 30, 12 0.3 0.81
Average distance to edge of clump 2.0 1.5 30, 12 −1.4 0.17
Average nest height 1.1 1.0 26, 29 −1.2 0.24
Average host height 4.7 4.6 26, 29 −0.3 0.80
Average distance to edge of host 0.8 0.4 26, 28 −2.0 10.06
Average distance to edge of clump 1.7 1.2 26, 28 −2.0 10.05
Table 24.    Least Bell’s Vireo nest characteristics and results of Student’s t-tests of successful versus unsuccessful nesting attempts at Seep and Reference sites, Marine Corps Base Camp Pendleton, 2021.
1

Significant results.

Table 25.    

Least Bell's Vireo nest characteristics and results of Student's t-tests of all nesting attempts at Seep versus Reference Sites, Marine Corps Base Camp Pendleton, 2021.

[n, number of nests in sample (Seep, Reference); t, Student’s t statistic; P, probability that the difference was not significant; m, meter]
Nest characteristics Seep sites Reference sites n t P
Average nest height (m) 1.0 1.1 41, 55 1.6 0.11
Average host height (m) 4.8 4.6 42, 55 −0.3 0.76
Average distance to edge of host (m) 0.9 0.6 42, 54 −2.1 10.04
Average distance to edge of clump (m) 1.8 1.4 42, 54 −1.9 10.06
Table 25.    Least Bell's Vireo nest characteristics and results of Student's t-tests of all nesting attempts at Seep versus Reference Sites, Marine Corps Base Camp Pendleton, 2021.
1

Significant results.

Discussion

Least Bell’s Vireo numbers have fluctuated during the past several years and manifested relatively consistently across several study areas in San Diego County, including MCBCP, the San Luis Rey River, the San Diego River, MCAS, and the Sweetwater Reservoir.

The range-wide vireo population gradually increased through the 1980s and 1990s, reaching a peak in 2009–10 before declining through 2012, fluctuating between 60 percent and 70 percent of peak numbers through 2017, then dramatically increasing in 2018, remaining high in 2019, increasing again in 2020, and dropping again in 2021 (B. Jones, unpub. data, 1985; Kus, 1989a, 1989b, 1991a, 1991b, 1993, 1995; Kus and Beck, 1998; B. Kus, U.S. Geological Survey, unpub. data, 2007–21; Allen and others, 2017, 2018; Ferree and Clark, 2018, 2019, 2020, 2021; Allen and Kus, 2019, 2020, 2021; Houston and others, 2021, 2022).

Between 2016 and 2018, the population trends at different study areas within the vireo’s range diverged, with vireos increasing on MCBCP from 2015 to 2016, but decreasing on MCAS and on the lower San Luis Rey River and remaining stable on the middle San Luis Rey River (B. Kus, U.S. Geological Survey, unpub. data, 2016). In 2017, there also was a discrepancy between sites, although in the opposite direction. By 2018, trends in vireo populations on MCBCP, the lower San Luis Rey River, the middle San Luis Rey (in areas not burned during a December 2017 fire), and at MCAS re-converged with a slight drop in 2019 and then a dramatic increase in 2020. From 2020 to 2021, vireo populations in these sites decreased again by 18 percent (MCBCP), 24 percent (lower San Luis Rey River; Houston and others, 2022), 6 percent (middle San Luis Rey River; Allen and others, 2017), and 44 percent (MCAS, although this decrease followed removal of approximately half of the vireo habitat in the MCAS study area between 2020 and 2021; Ferree and Clark, 2021). Although the population on Base dropped from 2020 to 2021, the 2021 population still remained the fourth highest count since 2005.

There was a general decrease in vireo numbers region-wide from 2010 to 2017 that can largely be attributed to drought conditions on the breeding grounds before and during that timeframe. Low precipitation likely compromised primary productivity, resulting in decreased annual plant and foliage growth. Consequently, foraging substrate and nesting cover for vireos likely decreased in extent and quality, affecting arthropod abundance and ultimately higher trophic level wildlife (vireos) that depend on these resources. Precipitation during 3 of the 5 bio-years since 2016 was 12–51 percent greater than the 2002–11 average (Office of Water Resources, 2021), likely positively affecting breeding productivity in 2017, 2019, and 2020 and driving an increase in the vireo population in 2018 and 2020. We expected that high breeding productivity in 2020 would lead to an increase in the number of vireo territories region-wide in 2021 as well; however, this did not occur. Instead, the vireo population on Base dropped, as reflected by the low adult survival from 2020 to 2021 (33 percent), which was lower than survival in any year since 2005 (range 44–77 percent). First-year survival from 2020 to 2021 (5 percent) also was lower than any year since 2005 (8–27 percent), although both first-year and adult survival may increase if additional banded birds that were missed in 2021 are detected in the future. Conversely, vireo breeding productivity was high in 2021 (3.5 young fledged per pair) compared to the average since 2005 (2.6 fledglings per pair) even though bio-year precipitation for 2020–21 was low (41 percent of the 2002–11 average).

We expected that soil moisture at the Seep sites would be higher than at the Reference sites, which have not received surface-water augmentation. However, there was no difference in soil moisture between Seep and Reference sites, nor was there a decreasing gradient in soil moisture with increasing distance from seep outlets. Precipitation in the 2020–21 bio-year was lower than average, and much of the natural surface water had dried up by the beginning of May, although there was a large pond that persisted until the end of June on the dirt road along the western border of the Pump Road sites. The seep pumps were intended to provide supplemental surface water to compensate for dry conditions, such as those that occurred during the 2021 breeding season. Surface-water and soil-moisture augmentation from the pumps in 2021 was limited to immediately surrounding the outlets. Other areas of high soil moisture occurred near the naturally ponded water at the western edge of the study sites. The documented higher soil moisture at Reference sites than at Seep sites in 2020 was driven by high soil moisture in the northern Pump Road monitoring site, which was a Reference site in 2020 until an artificial seep was installed for the 2021 field season. The northern Pump Road monitoring site (now Pump Road Seep site) had higher soil moisture than the three other monitoring sites at almost all sampling locations, continuing into 2021.

In 2021, we found a few differences in vegetation cover between the Seep and Reference sites. Seep sites had greater total foliage cover and greater native herbaceous cover below 2 m than Reference sites. Perhaps, related to greater foliage cover at Seep sites, Seep pairs had a higher proportion of eggs that ultimately fledged than did Reference pairs. The proportion of nests that successfully fledged young also was higher at Seep sites than Reference sites, although the number of pairs that fledged young and the number of young fledged per pair did not differ between sites. This discrepancy indicates that pairs at Reference sites had to build more nests and lay more eggs than pairs at Seep sites to produce a similar number of young. Daily nest survival also was higher at Seep sites than at Reference sites, supporting the proposition that Reference site pairs had to expend more energy to produce a comparable breeding output than did Seep site pairs. We did not find a relationship between soil moisture and total foliage cover or native herbaceous cover below 2 m. Therefore, it is likely that some other environmental factor, such as plant species composition, affected the volume of low canopy vegetation where vireos typically place their nests. At the Reference sites, all three of the most abundant herbaceous species were typical wetland species (horsetail fern [Equisetum spp.], sedge [Carex spp.], and common reed [Phragmites australis]), whereas only one of the top three herbaceous species at the Seep sites was a wetland species (bulrush [Scirpus spp.]). The other two of the top three herbaceous species at Seep sites were annuals that do not require wetland conditions (mugwort [Artemisia douglasiana] and western goldenrod [Euthamia occidentalis]). The architecture of wetland species, such as sedges, bulrush, and reeds, typically does not support as much horizontal leaf cover, and consequently, might not fill as much volume and provide as much cover as more xeric species, such as mugwort and goldenrod.

Tree canopy was higher at Reference sites than at Seep sites but was not related to soil moisture. The two direct ecological process that affect canopy height are vegetation growth (positive) and vegetation death (negative). We expect that increased soil moisture would benefit vegetation and cause increased vegetation growth, promoting a higher tree canopy at Seep sites and at plots with higher soil moisture over time, but this relationship was not clear in 2021.

In 2021, we continued to see vireos that originated outside of MCBCP moving onto Base and holding territories. One first-year vireo moved to MCBCP from the San Luis Rey River, where it hatched in 2020. Three 2-year-old vireos were detected on MCBCP in 2021 for the first time after they were banded on the San Luis Rey River in 2019. Conversely, one vireo that hatched on MCBCP in 2016 was detected off Base. These movements demonstrate the ability of vireos to disperse beyond their natal drainages. Incidental observations of vireos in areas that typically have not been thoroughly surveyed help to enhance our understanding of movements of both adult and dispersing first-year vireos. Further banding and resighting of vireos within southern California and Baja California Sur continues to increase our understanding of the extent of movement between populations and during migration and the role such movements play in maintaining genetic diversity and persistence in these populations. Continued monitoring of cohorts banded as nestlings provides the opportunity to collect lifetime reproductive data for a segment of the population, facilitating identification of age- and possibly sex-related patterns in life history characteristics that affect population size, productivity, and genetic structure.

Conclusions

Until 2011, the vireo population on Marine Corps Base Camp Pendleton (MCBCP or “Base”) tracked the overall increase in Least Bell’s Vireos in southern California since the late 1970s (U.S. Fish and Wildlife Service, 2006). Since its peak in 2010, the vireo population on MCBCP reached a 21-year low in 2015 and then rebounded in 2018 to the fifth highest count recorded. Extrapolating the number of vireos counted in core areas on MCBCP in 2020 to the entire Base, the Base-wide vireo population in 2020 was likely the highest on record. High productivity in 2017 and 2019 had a strong effect on the number of vireos that returned in 2018 and 2020. In 2021, the vireo population on base dropped again, although it still remained the fourth highest population since 2005.

The increasing trend in the vireo population in the 1980s and 1990s can largely be attributed to management actions, including control of Brown-headed Cowbirds and protection and restoration of riparian habitat. On MCBCP, annual Brown-headed Cowbird control has reduced cowbird parasitism to a negligible level since the mid-1990s, releasing a major limit on vireo breeding productivity. Although two vireo nests were parasitized by cowbirds in 2018, cowbird parasitism was not recorded on MCBCP in 2019, 2020, or 2021. In 2021, cowbird parasitism on the lower San Luis Rey River increased 400 percent from the parasitism rate in 2020 (B. Kus, U.S. Geological Survey, unpub. data, 2021), which is most likely because cowbird traps were not opened until April 28, 4 weeks after the traps were typically opened in the past (J. Sexton, T.W. Biological Services, written commun., 2021). Cowbird control has a demonstrably positive effect on vireo productivity (Kus, 1999; Kus and Whitfield, 2005) but must be consistently practiced to maintain the desired reduction in parasitism.

The recent fluctuations in the vireo population may be a consequence of a variety of interacting factors, including wildfire (affecting apparent population size, distribution, and habitat-related nesting productivity and predation), drought (affecting breeding productivity and survival), high floodwaters, and the inherent carrying capacity of the current habitat (breeding, migratory, or wintering). These factors are difficult to parse and are subject to change as a result of natural (for example, weather) and anthropogenic (for example, habitat alteration or restoration) processes, which makes future population trends difficult to predict.

The artificial seep pumps at the Old Treatment Ponds and at Pump Road did not have a noticeable effect on soil moisture in 2021; however, foliage cover below 4 meters was significantly greater at Seep sites, potentially contributing to higher breeding productivity (at the egg-scale) at Seep sites. The proximate effect of increased low canopy vegetation volume likely was a benefit to vireos; however, the direct effect of artificially increased surface water and soil moisture, which is a characteristic of high-quality Willow Flycatcher habitat, was lacking in 2021. The efficiency of artificial seep pumps may represent a temporal factor that will evolve in the future as conditions outside of the effect of the seep pumps become drier, allowing determination if the seep pumps can compensate for a lowered water table and less precipitation. The 2021 results of comparisons between Seep and Reference sites indicate that consideration of factors, such as topographic depressions, dry drainages, and less well-draining soil types, would be useful in deciding where to locate artificial seeps.

Although we did not document catastrophic wildfires on Base in 2021, continued vigilance is worthwhile for military and civilian activities on Base to avoid causing wildfires during the frequent high wildfire risk weather conditions in southern California. The wildfires that occurred in October 2013 and May 2014 were sparked by a combination of circumstances, including the ongoing drought, strong east winds that carried dry, hot air from the deserts, human activity, and electrical infrastructure failure as a result of strong winds (S. Sullivan, MCBCP, written commun., 2014). Other smaller fires on Base also have been ignited by military training involving the use of materials that can ignite fires (gunfire, vehicles with hot engines parked on dry grass) (S. Sullivan, MCBCP, written commun., 2014). Although most of these circumstances were beyond immediate human control, catastrophic events like wildfires highlight the delicate tipping point that can easily be upset by normally innocuous human actions. These effects can adversely affect vireo populations in the short term and potentially long term, causing direct mortality during the breeding season, destroying habitat during any time of the year, and possibly causing long-term changes in vegetation structure and composition and consequently reducing high quality breeding habitat for vireos.

Direct human impacts to vireo habitat were not documented in 2021, although continued attention to other potential impacts (weed control, off-road vehicle traffic) is warranted. Although some human impacts may be best mitigated by extreme action (closing high-speed roads in vireo habitat during vireo breeding season, prohibiting the use of firearms during dry, windy weather), other impacts may be mitigated by continued education and adjustments to schedules. Communication among personnel may reduce the instances of human-related impacts to vireos and occupied vireo habitat by allowing all participants to understand needs and flexibilities and adjust their activities accordingly. Our findings and experience indicate that impacts to vireos can be minimized when military training exercises and maintenance activities, such as continuing to clear vegetation outside the vireo breeding season or limiting these activities to areas not occupied by vireos, are coordinated among personnel. This coordination and cooperation among various departments could help maintain a balance between the sometimes-competing land uses on Base, including military activities, recreation, habitat protection, and endangered species management.

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Howell, S., Lynn, S., and Kus, B.E., 2020, Distribution, abundance, and breeding activities of the Southwestern Willow Flycatcher at Marine Corps Base Camp Pendleton, California—2019 annual data summary: Restricted-file federal interagency report, Prepared for Assistant Chief of Staff, Environmental Security, Marine Corps Base Camp Pendleton.

Kus, B.E., 1989a, Status and management of the Least Bell’s Vireo at the San Luis Rey River, San Diego County, California, 1988: Prepared for the for the State of California Department of Transportation, District 11. U.S. Geological Survey Report, 32 p. [Available at https://pubs.usgs.gov/publication/96319.]

Kus, B.E., 1989b, Status of the Least Bell’s Vireo at the San Luis Rey River, San Diego County, California 1989: Prepared for the State of California Department of Transportation, District 11, U.S. Geological Survey Report, 15 p. [Available at https://pubs.usgs.gov/publication/96327.]

Kus, B.E., 1991a, Status of the Least Bell’s Vireo at the San Luis Rey River, San Diego County, California 1990: Prepared for the State of California Department of Transportation, District 11, U.S. Geological Survey Report, 10 p. [Available at https://pubs.usgs.gov/publication/96803.]

Kus, B.E., 1991b, Distribution and breeding status of the Least Bell’s Vireo at the San Luis Rey River, San Diego County, California 1991: Prepared for the State of California Department of Transportation, District 11, U.S. Geological Survey Report, 15 p. [Available at https://pubs.usgs.gov/publication/96802.]

Kus, B.E., 1993, Distribution and breeding status of the Least Bell’s Vireo at the San Luis Rey River, San Diego County, California 1992–1993: Prepared for the State of California Department of Transportation, District 11, U.S. Geological Survey Report, 29 p. [Available at https://pubs.usgs.gov/publication/96644.]

Kus, B.E., 1995, Distribution and breeding status of the Least Bell’s Vireo at the San Luis Rey River, San Diego County, California 1994: Prepared for the State of California Department of Transportation, District 11, U.S. Geological Survey Report, 25 p. [Available at https://pubs.usgs.gov/publication/96591.]

Kus, B.E., 1998, Use of restored riparian habitat by the endangered Least Bell’s Vireo (Vireo bellii pusillus): Restoration Ecology, v. 6, no. 1, p. 75–82. [Available at https://doi.org/10.1046/j.1526-100x.1998.06110.x.]

Kus, B.E., 1999, Impacts of Brown-headed Cowbird parasitism on the productivity of the endangered Least Bell’s Vireo: Studies in Avian Biology, v. 18, p. 160–166.

Kus, B.E., and Beck, P.P., 1998, Distribution and abundance of the Least Bell’s Vireo (Vireo bellii pusillus) and the Southwestern Willow Flycatchers (Empidonax traillii extimus) at selected Southern California sites in 1997: Sacramento, Calif., Prepared for the California Department of Fish and Game, U.S. Geological Survey Report, 69 p. + append. [Available at https://www.researchgate.net/publication/260364086.]

Kus, B.E., and Whitfield, M.J., 2005, Parasitism, productivity, and population growth—Response of Least Bell’s Vireos (Vireo bellii pusillus) and Southwestern Willow Flycatchers (Empidonax traillii extimus) to cowbird (Molothrus spp.) control: Ornithological Monographs, v. 57, p. 16–27. [Available at https://doi.org/10.2307/40166811.]

Kus, B.E., Hopp, S.L., Johnson, R.R., and Brown, B.T., 2020, Bell's Vireo (Vireo bellii), version 1.0 in Poole, A.F., Poole, ed., Birds of the world: Ithaca, N.Y., Cornell Lab of Ornithology. [Available at https://doi.org/10.2173/bow.belvir.01.]

Laake, J.L., 2013, RMark—An R interface for analysis of capture-recapture data with MARK: Seattle, Wash., National Oceanic and Atmospheric Administration, Alaska Fisheries Science Center, National Marine Fisheries Service, Processed Report 2013-01, 25 p., accessed July 2, 2024, at https://www.researchgate.net/publication/267509042_RMark_an_R_Interface_for_analysis_of_capture-recapture_data_with_MARK.

Lynn, S., and Kus, B.E., 2009, Distribution, abundance and breeding activities of the Least Bell’s Vireo at Marine Corps Base Camp Pendleton, California—2008 annual report: Prepared for Assistant Chief of Staff, Environmental Security, Marine Corps Base Camp Pendleton.

Lynn, S., and Kus, B.E., 2010a, Distribution, abundance and breeding activities of the Least Bell’s Vireo at Marine Corps Base Camp Pendleton, California—2009 annual report: Prepared for Assistant Chief of Staff, Environmental Security, Marine Corps Base Camp Pendleton.

Lynn, S., and Kus, B.E., 2010b, Distribution, abundance and breeding activities of the Least Bell’s Vireo at Marine Corps Base Camp Pendleton, California—2010 annual report: Prepared for Assistant Chief of Staff, Environmental Security, Marine Corps Base Camp Pendleton.

Lynn, S., and Kus, B.E., 2011, Distribution, abundance and breeding activities of the Least Bell’s Vireo at Marine Corps Base Camp Pendleton, California—2011 annual report: Prepared for Assistant Chief of Staff, Environmental Security, Marine Corps Base Camp Pendleton.

Lynn, S., and Kus, B.E., 2012, Distribution, abundance and breeding activities of the Least Bell’s Vireo at Marine Corps Base Camp Pendleton, California—2012 annual report: Prepared for Assistant Chief of Staff, Environmental Security, Marine Corps Base Camp Pendleton.

Lynn, S., and Kus, B.E., 2013, Distribution, abundance and breeding activities of the Least Bell’s Vireo at Marine Corps Base Camp Pendleton, California—2013 annual report: Prepared for Assistant Chief of Staff, Environmental Security, Marine Corps Base Camp Pendleton.

Lynn, S., Allen, L.D., and Kus, B.E., 2014, Distribution, abundance, and breeding activities of the Least Bell’s Vireo at Marine Corps Base Camp Pendleton, California—2014 annual data summary: Prepared for Assistant Chief of Staff, Environmental Security, Marine Corps Base Camp Pendleton, U.S. Geological Survey Annual Data Summary, 118 p., accessed July 2, 2024, at https://sdmmp.com/view_article.php?cid=SDMMP_CID_187_659c77c0ba635.

Lynn, S., Allen, L.D., and Kus, B.E., 2015, Distribution, abundance, and breeding activities of the Least Bell’s Vireo at Marine Corps Base Camp Pendleton, California—2015 annual data summary: Prepared for Assistant Chief of Staff, Environmental Security, Marine Corps Base Camp Pendleton, U.S. Geological Survey annual Data Summary, 105 p., accessed July 2, 2024, at https://sdmmp.com/view_article.php?cid=SDMMP_CID_187_659c7a6f10976.

Lynn, S., Allen, L.D., and Kus, B.E., 2016, Distribution, abundance, and breeding activities of the Least Bell’s Vireo at Marine Corps Base Camp Pendleton, California—2016 annual data summary: Prepared for Assistant Chief of Staff, Environmental Security, Marine Corps Base Camp Pendleton, U.S. Geological Survey Annual Data Summary, 107 p., accessed July 2, 2024, at https://sdmmp.com/view_article.php?cid=SDMMP_CID_187_659c7c771dd08.

Lynn, S., Allen, L.D., and Kus, B.E., 2017, Distribution, abundance, and breeding activities of the Least Bell’s Vireo at Marine Corps Base Camp Pendleton, California—2017 annual data summary: Prepared for Assistant Chief of Staff, Environmental Security, Marine Corps Base Camp Pendleton, U.S. Geological Survey Annual Data Summary, 110 p., accessed July 2, 2024, at https://sdmmp.com/view_article.php?cid=SDMMP_CID_187_659c7fc35fb5d.

Lynn, S., Allen, L.D., and Kus, B.E., 2018, Distribution, abundance and breeding activities of the Least Bell’s Vireo at Marine Corps Base Camp Pendleton, California—2018 annual data summary: Restricted-file federal interagency report, Prepared for Assistant Chief of Staff, Environmental Security, Marine Corps Base Camp Pendleton.

Lynn, S., Allen, L.D., and Kus, B.E., 2020, Distribution, abundance and breeding activities of the Least Bell’s Vireo at Marine Corps Base Camp Pendleton, California—2019 annual data summary: Restricted-file federal interagency report, Prepared for Assistant Chief of Staff, Environmental Security, Marine Corps Base Camp Pendleton.

Lynn, S., Treadwell, M., and Kus, B.E., 2024, Distribution, abundance, and breeding activities of the Least Bell's Vireo at Marine Corps Base Camp Pendleton, California—2020 annual report: U.S. Geological Survey Open-File Report 2024–1009, 66 p.

Office of Water Resources, 2021, Lake O’Neill Station precipitation summary: Camp Pendleton, Calif., Prepared for the Assistant Chief of Staff, Facilities, Marine Corps Base Camp Pendleton.

R Core Team, 2022, R—A language and environment for statistical computing: Vienna, Austria, R Foundation for Statistical Computing, accessed September 15, 2023, at http://www.R-project.org.

Riparian Habitat Joint Venture, 2004, The riparian bird conservation plan—A strategy for reversing the decline of riparian associated birds in California, version 2.0: California Partners in Flight, accessed September 15, 2023, at https://www.usbr.gov/lc/socal/basinstudies/OWOWReferences/RiparianHabitat.pdf.

Rourke, J.W., and Kus, B.E., 2006, Distribution, abundance, and breeding activities of the Least Bell’s Vireo at Marine Corps Base Camp Pendleton, California—2005 annual report: Prepared for Assistant Chief of Staff, Environmental Security, Marine Corps Base Camp Pendleton, U.S. Geological Survey 2005 Annual Report, 50 p.

Rourke, J.W., and Kus, B.E., 2007, Distribution, abundance, and breeding activities of the Least Bell’s Vireo at Marine Corps Base Camp Pendleton, California—2006 annual data summary: Prepared for Assistant Chief of Staff, Environmental Security, Marine Corps Base Camp Pendleton.

Rourke, J.W., and Kus, B.E., 2008, Distribution, abundance, and breeding activities of the Least Bell’s Vireo at Marine Corps Base Camp Pendleton, California—2007 annual data summary: Prepared for Assistant Chief of Staff, Environmental Security, Marine Corps Base Camp Pendleton, U.S. Geological Survey Annual Data Summary, 68 p. [Available at https://sdmmp.com/view_article.php?cid=CID_ctamanah%40usgs.gov_5751c384040a9.]

Rourke, J.W., Kus, B.E., and Whitfield, M.J., 2004, Distribution and abundance of the Southwestern Willow Flycatcher at selected southern California sites in 2001: Prepared for the California Department of Fish and Game, Species Conservation and Recovery Program Report 2004–05, U.S. Geological Survey Final Report, 60 p. [Available at https://www.researchgate.net/publication/311067108.]

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U.S. Fish and Wildlife Service, 1986, Endangered and threatened wildlife and plants—Determination of endangered status for the Least Bell’s Vireo: Federal Register, v. 51, no. 85, p. 16474–16482.

U.S. Fish and Wildlife Service, 1995, Biological opinion (1-6-95-F-02)—Programmatic activities and conservation plans in riparian and estuarine/beach ecosystems on Marine Corps Base, Camp Pendleton: U.S. Fish and Wildlife Service, Carlsbad Field Office, 270 p.

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White, G.C., and Burnham, K.P., 1999, Program MARK—Survival estimation from populations of marked animals: Bird Study, v. 46, no. sup1, p. S120–S139. [Available at https://doi.org/10.1080/00063659909477239.]

Appendix 1. Least Bell’s Vireo Survey Areas at Marine Corps Base Camp Pendleton, 2021

1.1. Topographic map showing colored polygons demarcating survey areas.
Figure 1.1.

Least Bell’s Vireo survey areas at Marine Corps Base Camp Pendleton, 2021: Upper Santa Margarita River, Fallbrook Creek, Lake O’Neill, De Luz Creek, Roblar Creek, and Basilone and Roblar Roads. Core areas and Group C areas were surveyed in 2021.

1.2. Topographic map showing colored polygons demarcating survey areas.
Figure 1.2.

Least Bell’s Vireo survey areas at Marine Corps Base Camp Pendleton, 2021: Lower Santa Margarita River, 22 Area, Pueblitos Canyon, Tuley Canyon, Newton Canyon, Cockleburr Canyon, French Creek, and Aliso Creek.

1.3. Topographic map showing colored polygons demarcating survey areas.
Figure 1.3.

Least Bell’s Vireo survey areas at Marine Corps Base Camp Pendleton, 2021: San Onofre Creek South Fork, Ammunition Supply Point, Horno Canyon, Piedra de Lumbre Creek, Las Flores Creek, and Hidden Canyon.

1.4. Topographic map showing colored polygons demarcating survey areas.
Figure 1.4.

Least Bell’s Vireo survey areas at Marine Corps Base Camp Pendleton, 2021: Talega Canyon, Cristianitos Creek, San Mateo Creek, and San Onofre Creek.

1.5. Topographic map showing colored polygons demarcating survey areas.
Figure 1.5.

Least Bell’s Vireo survey areas at Marine Corps Base Camp Pendleton, 2021: Upper San Mateo Creek.

1.6. Topographic map showing colored polygons demarcating survey areas.
Figure 1.6.

Least Bell’s Vireo survey areas at Marine Corps Base Camp Pendleton, 2021: Windmill Canyon, Ysidora Basin to Windmill Canyon, Pilgrim Creek, and De Luz Homes Habitat.

Appendix 2. Vegetation Sampling Locations and Vegetation Sampling Data Sheet, Marine Corps Base Camp Pendleton, 2021

Table 2.1.    

Vegetation sampling locations and vegetation sampling data sheet, Marine Corps Base Camp Pendleton, 2021.

[WGS 84, World Geodetic System of 1984]
Territory Site Longitude Latitude Datum
ACA Reference −117.372286 33.270875 WGS 84
ALN Reference −117.377681 33.270004 WGS 84
AND Treatment −117.380140 33.281929 WGS 84
ARW Treatment −117.377398 33.284092 WGS 84
BGT Reference −117.378402 33.27098 WGS 84
BAX Reference −117.373217 33.270878 WGS 84
BOR Reference −117.374614 33.266378 WGS 84
CAP Reference −117.373562 33.266963 WGS 84
CEL Treatment −117.378145 33.285929 WGS 84
CHW Treatment −117.374394 33.280879 WGS 84
CLM Reference −117.380726 33.271391 WGS 84
CRA Reference −117.374852 33.270433 WGS 84
DOO Treatment −117.373103 33.280825 WGS 84
ELR Treatment −117.380731 33.285253 WGS 84
ENC Reference −117.380966 33.272063 WGS 84
HDO Treatment −117.374214 33.278930 WGS 84
FRX Reference −117.377731 33.270615 WGS 84
FRO Treatment −117.378960 33.281907 WGS 84
GAN Treatment −117.378631 33.284427 WGS 84
GIM Treatment −117.379554 33.285856 WGS 84
HAS Treatment −117.378400 33.283079 WGS 84
JAC Reference −117.374842 33.26773 WGS 84
JOS Reference −177.37344 33.270119 WGS 84
KTM Reference −117.372696 33.270233 WGS 84
KNO Treatment −117.374049 33.280596 WGS 84
KYL Treatment −117.374689 33.283310 WGS 84
LEG Treatment −117.380450 33.283831 WGS 84
LEI Treatment −117.373381 33.281438 WGS 84
MAL Reference −117.379957 33.273695 WGS 84
MRY Treatment −117.377243 33.282331 WGS 84
PAP Treatment −117.373304 33.282735 WGS 84
PNA Reference −117.373736 33.267749 WGS 84
PIP Treatment −117.378039 33.281584 WGS 84
REY Treatment −117.375490 33.281926 WGS 84
RHI Reference −117.379378 33.274622 WGS 84
ROK Reference −117.375706 33.27036 WGS 84
RUB Reference −117.381496 33.270617 WGS 84
SAG Reference −117.375813 33.268448 WGS 84
SLX Reference −117.380534 33.272905 WGS 84
CSAL Reference −117.378264 33.273269 WGS 84
SAM Reference −117.378071 33.274307 WGS 84
SEQ Reference −117.374861 33.267322 WGS 84
SKY Treatment −117.373780 33.282323 WGS 84
SNK Treatment −117.374902 33.279887 WGS 84
SLO Treatment −117.375996 33.281181 WGS 84
TRE Treatment −117.378682 33.282535 WGS 84
VIT Reference −117.378333 33.270348 WGS 84
YOD Treatment −117.375238 33.280812 WGS 84
Table 2.1.    Vegetation sampling locations and vegetation sampling data sheet, Marine Corps Base Camp Pendleton, 2021.
2.1. Blank grid of squares for entering plant species and percentage cover at each height category, first two plots.
Figure 2.1.

Pendleton Seep vegetation data form, page 1-2021.

2.2. Blank grid of squares for entering plant species and percentage cover at each height category, second two plots.
Figure 2.2.

Pendleton Seep vegetation data form, page 2–2021

Appendix 3. Locations of Least Bell’s Vireos at Marine Corps Base Camp Pendleton, 2021

3.1. Topographic map showing colored markers for vireo territories.
Figure 3.1.

Locations of Least Bell’s Vireos at Marine Corps Base Camp Pendleton, 2021: De Luz Creek.

3.2. Topographic map showing colored markers for vireo territories.
Figure 3.2.

Locations of Least Bell’s Vireos at Marine Corps Base Camp Pendleton, 2021: Santa Margarita River and Lake O’Neill.

3.3. Topographic map showing colored markers for vireo territories.
Figure 3.3.

Locations of Least Bell’s Vireos at Marine Corps Base Camp Pendleton, 2021: upper middle section of the Santa Margarita River.

3.4. Topographic map showing colored markers for vireo territories.
Figure 3.4.

Locations of Least Bell’s Vireos at Marine Corps Base Camp Pendleton, 2021: lower middle section of the Santa Margarita River.

3.5. Topographic map showing colored markers for vireo territories.
Figure 3.5.

Locations of Least Bell’s Vireos at Marine Corps Base Camp Pendleton, 2021: Santa Margarita River and Tuley Canyon.

3.6. Topographic map showing colored markers for vireo territories.
Figure 3.6.

Locations of Least Bell’s Vireos at Marine Corps Base Camp Pendleton, 2021: Santa Margarita River and French Canyon.

3.7. Topographic map showing colored markers for vireo territories.
Figure 3.7.

Locations of Least Bell’s Vireos at Marine Corps Base Camp Pendleton, 2021: Lower Pilgrim Creek.

3.8. Topographic map showing colored markers for vireo territories.
Figure 3.8.

Locations of Least Bell’s Vireos at Marine Corps Base Camp Pendleton, 2021: Aliso Creek and French Canyon.

3.9. Topographic map showing colored markers for vireo territories.
Figure 3.9.

Locations of Least Bell’s Vireos at Marine Corps Base Camp Pendleton, 2021: Lower Las Flores Creek.

3.10. Topographic map showing colored markers for vireo territories.
Figure 3.10.

Locations of Least Bell’s Vireos at Marine Corps Base Camp Pendleton, 2021: Upper Las Flores Creek.

3.11. Topographic map showing colored markers for vireo territories.
Figure 3.11.

Locations of Least Bell’s Vireos at Marine Corps Base Camp Pendleton, 2021: Lower San Onofre Creek and Lower San Mateo Creek.

3.12. Topographic map showing colored markers for vireo territories.
Figure 3.12.

Locations of Least Bell’s Vireos at Marine Corps Base Camp Pendleton, 2021: San Onofre Creek (West).

3.13. Topographic map showing colored markers for vireo territories.
Figure 3.13.

Locations of Least Bell’s Vireos at Marine Corps Base Camp Pendleton, 2021: Upper San Mateo Creek.

Appendix 4. Banded Least Bell’s Vireos at Marine Corps Base Camp Pendleton, 2021

Table 4.1.    

Banded Least Bell’s Vireos at Marine Corps Base Camp Pendleton, 2021.

[Band colors: OROR, plastic orange; PUOR, plastic purple-orange split; Mgo, gold numbered federal band; BPST, plastic black-pink striped; YEYE, plastic yellow; Msi, silver numbered federal band; BKYE, plastic black-yellow split; PUPU, plastic purple; Mdb, dark blue numbered federal band; BWST, plastic dark blue-white striped; ORDG, plastic orange-dark green split; WHPU, plastic white-purple split; YEPU, plastic yellow-purple split; DPWH, plastic dark pink-white split; DPDP, plastic dark pink; WHDP, plastic white-dark pink split; ORPU, plastic orange-purple split; BKBK, plastic black; PUYE, plastic purple-yellow split; DGOR, plastic dark green-orange split; BYST, plastic black-yellow striped; PUWH, plastic purple-white split; YEBK, plastic yellow-black split; WHWH, plastic white; pupu, metal purple. Sex: F, Female; M, Male. Location codes in comments: SLR, San Luis Rey River; SM MAPS, Santa Margarita MAPS Station; SMR, Santa Margarita River; DL MAPS, De Luz MAPS. All other 3-letter codes are territory designations. Abbreviations: ≥, greater than or equal to; yr(s), year(s); —, no bands]
Sex Band combination Age Comments
Left leg Right leg
M OROR PUOR Mgo ≥5 yrs Banded an adult at LEM in 2017.
M BPST YEYE Mgo 4 yrs Banded a nestling at PEP in 2017.
M — Msi ≥1 yr Banded at unknown age, location, and banding year.
F Mgo — ≥1 yr Banded at unknown age, location, and banding year.
M BKYE Msi — 2 yrs Banded a nestling on the SLR in 2019.
M BPST PUPU Mdb 2 yrs Banded a nestling on the SLR in 2019.
M PUPU Mdb BWST 1 yr Banded a nestling on the SLR in 2020.
M BPST BWST Mdb 1 yr Banded as a nestling on the SLR in 2019.
M ORDG PUOR Mgo ≥7 yrs Banded an adult at UNI in 2015.
M WHPU Mgo — ≥7 yrs Banded an adult at HOL in 2015.
M YEPU DPWH Mgo 6 yrs Banded a nestling at LUC in 2015.
F BPST DPDP Mgo 6 yrs Banded a nestling at ARI in 2015.
M WHDP ORPU Mgo 6 yrs Banded a nestling at HDX in 2015.
M ORDG BKBK Mgo 6 yrs Banded a nestling at KNG in 2015.
M ORDG YEYE Mgo 6 yrs Banded a nestling at KOA in 2015.
M YEYE Mgo ≥5 yrs Banded an adult at SM MAPS in 2017.
M PUYE PUOR Mgo ≥5 yrs Banded an adult at SM MAPS in 2017.
M DPWH Mgo DGOR 5 yrs Banded a nestling at UNI in 2016.
F BPST BKYE Mgo ≥4 yrs Banded an adult at SM MAPS in 2018.
M DPDP Mdb WHPU 4 yrs Banded an adult on the SLR in 2018.
M BYST PUWH Mgo 4 yrs Banded a nestling at ZAM in 2017.
M BPST PUOR Mgo ≥3 yrs Banded an adult at MER in 2019.
M WHPU WHPU Mgo ≥3 yrs Banded an adult at HLD in 2019.
F BKYE Mgo DPDP ≥3 yrs Banded an adult at SM MAPS in 2019.
M PUPU BKYE Mgo ≥3 yrs Banded an adult at RNR in 2019.
M WHPU YEBK Mgo 3 yrs Banded an adult at ARL in 2019.
M PUOR BKYE Mgo 3 yrs Banded an adult at SM MAPS in 2019.
M WHWH Mgo WHDP ≥2 yrs Banded an adult at CHW in 2020.
M ORDG Mgo ORDG ≥2 yrs Banded an adult at KTM in 2020.
M WHWH Mgo pupu ≥2 yrs Banded an adult at ACA in 2020.
M Mgo ORPU ≥2 yrs Banded an adult at CRA in 2020.
M YEYE Mgo pupu ≥2 yrs Banded an adult at KYL in 2020.
M PUWH Mgo BKYE ≥2 yrs Banded an adult at SLO in 2020.
M OROR Mgo BPST ≥2 yrs Banded an adult at KNO in 2020.
F Mgo YEYE pupu ≥2 yrs Banded an adult at FRO in 2020.
M ORDG Mgo YEPU ≥2 yrs Banded an adult at MAL in 2020.
M PUYE Mgo WHPU ≥2 yrs Banded an adult at SLX in 2020.
M pupu PUOR Mgo ≥2 yrs Banded an adult at AMO in 2020.
M PUOR Mgo YEPU ≥2 yrs Banded an adult at GAL in 2020.
M Mgo PUPU ≥2 yrs Banded an adult at GAN in 2020.
F pupu BKYE Mgo ≥2 yrs Banded an adult at MRY in 2020.
M — YEPU Mgo ≥2 yrs Banded an adult at PIP in 2020.
M PUYE DPWH Mgo 2 yrs Banded a juvenile at SM MAPS in 2019.
M WHDP Mgo PUYE 2 yrs Banded a juvenile at DL MAPS in 2019.
F PUWH BKYE Mgo 2 yrs Banded a nestling at HOU in 2019.
M YEPU WHWH Mgo 2 yrs Banded a nestling at WOM in 2019.
M PUYE BKBK 2 yrs Banded a nestling at LEM in 2019.
M WHWH pupu Mgo 2 yrs Banded a nestling at ODN in 2019.
M DPWH ORDG Mgo 2 yrs Banded a nestling at TEN in 2019.
M WHPU WHDP Mdb 2 yrs Banded a nestling on the SLR in 2019.
F WHDP pupu Mdb 2 yrs Banded a nestling on the SLR in 2019.
M DPDP pupu Mgo ≥1 yr Banded an adult at VIT in 2021.
M YEYE BKYE Mgo ≥1 yr Banded an adult at CLM in 2021.
M BKBK Mgo pupu ≥1 yr Banded an adult at GIM in 2021.
M Mgo — ≥1 yr Banded at unknown age, on the SMR before 2021.
M — Msi ≥1 yr Banded at unknown age, location, and banding year.
M Msi — ≥1 yr Banded at unknown age, location, and banding year.
M — Msi ≥1 yr Banded at unknown age, location, and banding year.
M — Msi ≥1 yr Banded at unknown age, location, and banding year.
M — Msi ≥1 yr Banded at unknown age, location, and banding year.
M — Msi ≥1 yr Banded at unknown age, location, and banding year.
M — Msi ≥1 yr Banded at unknown age, location, and banding year.
M — Msi ≥1 yr Banded at unknown age, location, and banding year.
M Msi — ≥1 yr Banded at unknown age, location, and banding year.
F — Mdb ≥1 yr Banded at age unknown on the SLR before 2020.
F — Mdb ≥1 yr Banded at age unknown on the SLR before 2020.
F Msi — ≥1 yr Banded at unknown age, location, and banding year.
F — Msi ≥1 yr Banded at unknown age, location, and banding year.
F — Msi ≥1 yr Banded at unknown age, location, and banding year.
F — Msi ≥1 yr Banded at unknown age, location, and banding year.
F — Msi ≥1 yr Banded at unknown age, location, and banding year.
F — Mgo ≥1 yr Banded at unknown age, location before 2020.
F — Msi ≥1 yr Banded at unknown age, location, and banding year.
F — Msi ≥1 yr Banded at unknown age, location, and banding year.
F Msi — ≥1 yr Banded at unknown age, location, and banding year.
F Unknown Unknown ≥1 yr Banded at unknown age, location before 2021.
F — Msi ≥1 yr Banded at unknown age, location, and banding year.
F DPWH Mgo YEYE 1 yr Banded a nestling at KTM in 2020.
M WHDP Mgo DGOR 1 yr Banded a nestling at KTM in 2020.
M OROR Mgo DPWH 1 yr Banded a nestling at TET in 2020.
M WHPU Mgo PUOR 1 yr Banded a nestling at KYL in 2020.
Table 4.1.    Banded Least Bell’s Vireos at Marine Corps Base Camp Pendleton, 2021.

Appendix 5. Between-Year Movement of Adult Least Bell’s Vireos at Marine Corps Base Camp Pendleton, 2021

Table 5.1.    

Between-Year movement of adult Least Bell’s Vireos at Marine Corps Base Camp Pendleton, 2021.

[Drainage Codes: SMR, Santa Margarita River; DL, De Luz Creek; PL, Pilgrim Creek; SLR, San Luis Rey River; FR, French Creek; SM MAPS, Santa Margarita MAPS Station. Band colors: YEPU, plastic yellow-purple split; Mgo, gold numbered federal band; pupu, metal purple; YEYE, plastic yellow; PUOR, plastic purple-orange split; BKBK, plastic black; BKYE, plastic black-yellow split; WHPU, plastic white-purple split; YEBK, plastic yellow-black split; OROR, plastic orange; DPWH, plastic dark pink-white split; DGOR, plastic dark green-orange split; BPST, plastic black-pink striped; PUPU, plastic purple; ORPU, plastic orange-purple split; Mdb, dark blue numbered federal band; ORDG, plastic orange-dark green split; WHWH, plastic white; WHDP, plastic white-dark pink split; BYST, plastic black-yellow striped; PUWH, plastic purple-white split; PUYE, plastic purple-yellow split; DPDP, plastic dark pink; Msi, silver numbered federal band. Sex: F, female; M, male. Abbreviations: km, kilometers; —, no bands; ≥, greater than or equal to; yr(s), year(s)]
Drainage/Territory Distance
moved
(km)		 Band combination Age
in 2021		 Sex
Last seen 2021 Left leg Right leg
SMR/PIP SMR/PIP 0.0 — YEPU Mgo ≥2 yrs M
SMR/GAN SMR/GAN 0.0 Mgo pupu ≥2 yrs M
SMR/KYL SMR/KYL 0.0 YEYE Mgo pupu ≥2 yrs M
SMR/PNA SMR/PNA 0.0 PUOR BKYE Mgo 3 yrs M
SMR/AH08 SMR/AH05 0.0 WHPU YEBK Mgo 3 yrs M
DL/DS15 DL/DS15 0.0 OROR PUOR Mgo ≥5 yrs M
SMR/AH38 SMR/AH17 0.0 DPWH Mgo DGOR 5 yrs M
SMR/HE41 SMR/HE18 0.0 WHPU Mgo — ≥7 yrs M
SMR/KNO SMR/KNO 0.0 OROR Mgo BPST ≥2 yrs M
SMR/CRA SMR/CRA 0.0 Mgo ORPU ≥2 yrs M
PL/PS05 PL/PS15 0.0 BPST PUPU Mdb ≥2 yrs M
SMR/KTM SMR/KTM 0.0 ORDG Mgo ORDG ≥2 yrs M
SMR/ACA SMR/ACA 0.0 WHWH Mgo pupu ≥2 yrs M
SMR/HE06 SMR/HE08 0.0 BPST PUOR Mgo ≥3 yrs M
SMR/AH10 SMR/AH09 0.0 pupu BKYE Mgo ≥3 yrs M
SMR/HW27 SMR/HW19 0.0 WHDP ORPU Mgo 6 yrs M
SMR/HE57 SMR/HE04 0.0 BYST PUWH Mgo 4 yrs M
SMR/SLX SMR/SLX 0.0 PUYE Mgo WHPU ≥2 yrs M
SMR/HW45 SMR/HW34 0.0 ORDG YEYE Mgo 6 yrs M
SMR/AE03 SMR/AE22 0.0 PUYE DPWH Mgo ≥2 yrs M
SMR/SLO SMR/SLO 0.0 PUWH Mgo BKYE ≥2 yrs M
SMR/CHW SMR/CHW 0.0 WHWH Mgo WHDP ≥2 yrs M
SMR/AH19 SMR/AH02 0.0 ORDG PUOR Mgo ≥7 yrs M
SMR/REY SMR/REY 0.0 ORDG BKBK Mgo 6 yrs M
SMR/MAL SMR/RHI 0.0 ORDG Mgo YEPU ≥2 yrs M
SMR/AMO SMR/AMO 0.0 pupu PUOR Mgo ≥2 yrs M
SMR/HE40 SMR/HE66 0.1 PUYE BKBK ≥2 yrs M
SMR/BN18 SMR/BN25 0.1 YEYE Mgo ≥5 yrs M
SMR/BN20 SMR/BN04 0.1 PUYE PUOR Mgo ≥5 yrs M
SMR/HW66 SMR/HW13 0.1 DPWH ORDG Mgo ≥2 yrs M
SMR/HE03 SMR/HE11 0.1 WHPU WHPU Mgo ≥3 yrs M
SMR/GAL SMR/TRE 0.1 PUOR Mgo YEPU ≥2 yrs M
SMR/CON SMR/PNA 0.1 BKYE Mgo DPDP ≥3 yrs F
SMR/RR15 SMR/RR05 0.1 YEPU DPWH Mgo 6 yrs M
SMR/FRO SMR/LEG 0.3 Mgo YEYE pupu ≥2 yrs F
SMR/MRY SMR/CHW 0.3 pupu BKYE Mgo ≥2 yrs F
SMR/HE72 SMR/HE37 1.1 BPST DPDP Mgo 6 yrs F
SMR/AR02 SMR/AR02 0.0 DPDP Mdb WHPU 4 yrs M
DL/LEM DL/DS18 0.0 BPST YEYE Mgo 4 yrs M
SMR/ODN SMR/HE41 0.7 WHWH pupu Mgo ≥2 yrs M
SMR/HOU SMR/HE26 0.8 PUWH BKYE Mgo ≥2 yrs F
SLR/WANI SMR/RHI 5.7 WHDP pupu Mdb ≥2 yrs F
SLR/DDOL SMR/HE63 8.0 WHPU WHDP Mdb ≥2 yrs M
SLR/FO1 FR/FR01 9.1 BKYE Msi — ≥2 yrs M
SMR/WOM SMR/ES34 11.0 YEPU WHWH Mgo ≥2 yrs M
DL/DL MAPS SMR/ALN 13.7 WHDP Mgo PUYE ≥2 yrs M
SMR/SM MAPS SMR/ROK 0.4 BPST BKYE Mgo ≥4 yrs F
SMR/SIL SLR/BHAZ 12.7 YEBK Mgo DGOR 5 yrs M
Table 5.1.    Between-Year movement of adult Least Bell’s Vireos at Marine Corps Base Camp Pendleton, 2021.

Appendix 6. Status and Nesting Activities of Least Bell's Vireos at Marine Corps Base Camp Pendleton, 2021

Table 6.1.    

Status and nesting activities of Least Bell's Vireos at Marine Corps Base Camp Pendleton, 2021.

[#, number; F, fully monitored territory; SUC, fledged at least one Least Bell’s Vireo young; —, none; PRE, nest failure caused by predation; INC, nest not completed; UNK, reason for nest failure/abandonment unknown; FAL, false nest, built by male; OTH, nest failed with known cause other than predation or parasitism]
Territory Nest Monitoring Nest
fate		 #
Fledged		 Comments
AND 1 F SUC 3 —
ARW 1 F PRE — Failed with nestlings.
ARW 2 F SUC 3 —
CEL 1 F PRE — Failed with nestlings.
CHW 1 F PRE — Failed with nestlings.
CHW 2 F SUC 2 One egg did not hatch.
DOO 1 F SUC 4 —
DOO 2 F SUC 3 —
ELR 1 F SUC 3 —
FRO 1 F SUC 2 —
GAN 1 F INC — —
GAN 2 F SUC 3 —
GAN 3 F SUC 3 —
GIM 1 F PRE — Failed with nestlings.
GIM 2 F PRE — Failed with nestlings.
HAS 1 F SUC 3 —
HDO 1 F PRE — One egg or nestling disappeared; failed with nestlings.
HDO 2 F PRE — One nestling disappeared prior to nest failure; failed with nestlings.
KNO 1 F SUC 2 One egg disappeared.
KNO 2 F PRE — One egg or nestling disappeared. Female found dead below failed nest. Nest failed with nestlings.
KYL 1 F PRE — Failed with nestlings.
KYL 2 F SUC 4 —
LEG 1 F SUC 4 —
LEI 1 F SUC 4 —
LEI 2 F SUC 4 —
MRY 1 F SUC 3 One egg disappeared.
MRY 2 F SUC 2 —
PAP 1 F SUC 3 One egg did not hatch.
PAP 2 F SUC 2 One egg did not hatch.
PIP 1 F INC — —
PIP 2 F SUC 3 —
PIP 3 F SUC 3 —
REY 1 F SUC 3 One egg or nestling disappeared.
REY 2 F SUC 4 —
SKY 1 F PRE — Failed with nestlings.
SKY 2 F SUC 3 —
SLO 1 F SUC 3 —
SNK 1 F SUC 2 One nestling disappeared.
TRE 1 F SUC 3 —
TRE 2 F SUC 3 —
YOD 1 F SUC 3 —
YOD 2 F SUC 2 One egg or nestling disappeared.
ACA 1 F INC — —
ACA 2 F UNK — Failed before eggs were confirmed.
ACA 3 F PRE — Failed with eggs.
ACA 4 F PRE — Failed with eggs.
ACA 5 F PRE — Failed with nestlings.
ALN 1 F SUC 3 —
BAX 1 F SUC 3 One egg or nestling disappeared.
BGT 1 F SUC 4 —
BGT 2 F UNK — Failed before eggs were confirmed. Rodent feces found in nest.
BGT 3 F SUC 2 —
BOR 1 F PRE — Failed with nestlings.
BOR 2 F SUC 3 —
CAP 1 F PRE — Failed with punctured eggs.
CAP 2 F SUC 1 Two eggs or nestlings disappeared.
CLM 1 F PRE — Failed with nestlings.
CLM 2 F PRE — One egg disappeared; failed with nestlings.
CLM 3 F SUC 3 —
CRA 1 F SUC 2 Two nestlings disappeared.
CRX 1 F FAL — Nest building by single male who disappeared shortly after nest was discovered.
CSAL 1 F PRE — Failed with eggs.
CSAL 2 F INC — —
CSAL 3 F SUC 3 —
ENC 1 F INC — —
ENC 2 F SUC 4 —
FRX 1 F SUC 4 —
FRX 2 F PRE — Failed with eggs.
FRX 3 F SUC 3 —
JAC 1 F SUC 4 —
JAC 2 F SUC 3 —
JOS 1 F SUC 3 One egg or nestling disappeared.
JOS 2 F PRE — One egg did not hatch; failed with nestlings.
KTM 1 F SUC 4 —
KTM 2 F SUC 4 —
MAL 1 F PRE — Failed with eggs.
MAL 2 F PRE — Failed with eggs.
PNA 1 F PRE — One egg or nestling disappeared; failed with nestlings.
PNA 2 F SUC 2 Two eggs or nestlings disappeared.
RHI 1 F SUC 3 —
ROK 1 F SUC 3 —
ROK 2 F SUC 4 —
RUB 1 F INC — —
RUB 2 F SUC 3 —
SAG 1 F PRE — One egg disappeared; failed with nestlings.
SAG 2 F UNK — Failed before eggs were confirmed.
SAG 3 F PRE — Failed with eggs.
SAG 4 F OTH — Eggs did not develop, may have been infertile.
SAM 1 F SUC 4 —
SEQ 1 F SUC 3 —
SLX 1 F INC — —
SLX 2 F PRE — Failed with eggs.
SLX 3 F PRE — Failed with nestlings.
SLX 4 F SUC 3 —
VIT 1 F PRE — Failed with eggs.
VIT 2 F PRE — Failed with eggs.
VIT 3 F SUC 3 —
Table 6.1.    Status and nesting activities of Least Bell's Vireos at Marine Corps Base Camp Pendleton, 2021.

Conversion Factors

International System of Units to U.S. customary units

Multiply By To obtain
meter (m) 3.281 foot (ft)
kilometer (km) 0.6214 mile (mi)
square meter (m2) 10.76 square foot (ft2)
hectare (ha) 2.471 acre (ac)
kilometer per hour (km/h) 0.6214 mile per hour (mi/h) 
liters per minute (l/min) 0.2642 gallons per second (g/min)

Datum

Horizontal coordinate information in text is referenced to the World Geodetic System of 1984 (WGS 84).

Horizontal coordinate information in mapped figures is referenced to the North American Datum of 1983 (NAD 83).

Abbreviations

AIC

Akaike’s Information Criterion

AICc

Akaike’s Information Criterion for small sample sizes

DSR

daily survival rate

MAPS

Monitoring Avian Productivity and Survivorship

MCAS

Marine Corps Air Station, Camp Pendleton

MCBCP

Marine Corps Base Camp Pendleton

ΔAICc

difference in AICc

For more information concerning the research in this report, contact the

Director, Western Ecological Research Center

U.S. Geological Survey

3020 State University Drive East

Sacramento, California 95819

https://www.usgs.gov/centers/werc

Publishing support provided by the U.S. Geological Survey

Science Publishing Network, Sacramento Publishing Service Center

Disclaimers

Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

Suggested Citation

Lynn, S., Treadwell, M., and Kus, B.E., 2024, Distribution, abundance, and breeding activities of the Least Bell's Vireo at Marine Corps Base Camp Pendleton, California—2021 annual report: U.S. Geological Survey Open-File Report 2023–1096, 68 p., https://doi.org/​10.3133/​ofr20231096.

ISSN: 2331-1258 (online)

Publication type Report
Publication Subtype USGS Numbered Series
Title Distribution, abundance, and breeding activities of the Least Bell's Vireo at Marine Corps Base Camp Pendleton, California—2021 annual report
Series title Open-File Report
Series number 2023-1096
DOI 10.3133/ofr20231096
Year Published 2024
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Western Ecological Research Center
Description ix, 68 p.
Online Only (Y/N) Y
Google Analytic Metrics Metrics page
Additional publication details