The Least Bell’s Vireo (Vireo bellii pusillus; 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 contributing to the vireo's decline have been the expansion in range of the brood-parasitic Brown-headed Cowbird (Molothrus ater; cowbird), to include the Pacific Coast (U.S. Fish and Wildlife Service, 1986; Franzreb, 1989; Kus, 1998, 1999; Kus and others, 2010), and the introduction of invasive exotic plant species such as giant reed (Arundo donax) into riparian systems. By 1986, the vireo population in California numbered just 300 territorial males (U.S. Fish and Wildlife Service, 1986).

In response to the dramatic reduction in numbers of vireos in California, the California Fish and Game Commission listed the species as endangered in 1980, with the U.S. Fish and Wildlife Service (USFWS) following 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, 1999; Kus and Whitfield, 2005). As of 2006, the statewide vireo population was estimated to be approximately 2,500–3,000 territories (U.S. Fish and Wildlife Service, 2006a), of which approximately 10 percent occurred along the San Luis Rey River between Interstate 15 and Interstate 5.

Male vireos arrive on breeding grounds in southern California in mid-March. Male vireos are vocally conspicuous and frequently sing their diagnostic primary song from exposed perches throughout the breeding season. Females arrive approximately 1–2 weeks after males and are more secretive. They often are seen early in the season traveling through habitat with the male. 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 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 re-nest after a failed attempt provided ample time remains within the breeding season. Vireos rarely fledge more than one brood in a season, although double-brooding can be more common during years when breeding conditions are favorable (early initiation, high early fledging success; Ferree and Kus, unpub. data, 2008b; Houston and others, unpub. data, 2017, 2019). Nesting lasts from early April through July, but adults and juvenile birds remain on the breeding grounds into late September through early October before migrating to their wintering grounds in southern Baja California, Mexico (Kus and others, 2010).

The Southwestern Willow Flycatcher (Empidonax traillii extimus; flycatcher) is one of four subspecies of Willow Flycatcher in the United States, with a breeding range including southern California, Arizona, New Mexico, extreme southern portions of Nevada and Utah, and western Texas (Hubbard, 1987; Unitt, 1987). Restricted to riparian habitat for breeding, the flycatcher has declined in recent decades in response to widespread habitat loss throughout its range and, possibly, cowbird parasitism (Wheelock, 1912; Willett, 1933; Grinnell and Miller, 1944; Remsen, 1978; Garrett and Dunn, 1981; Unitt, 1984, 1987; Gaines, 1988; Schlorff, 1990; Whitfield and Sogge, 1999; Kus and Whitfield, 2005). By 1993, the species was believed to number approximately 70 pairs in California (U.S. Fish and Wildlife Service, 1993) in small, disjunct populations. The flycatcher was listed as endangered by the state of California in 1992 and by the USFWS in 1995.

Willow Flycatchers in southern California co-occur with the vireo. However, unlike the vireo, which has increased tenfold since the mid-1980s in response to management efforts, Willow Flycatcher numbers have remained low. Currently, most flycatchers in California are concentrated in two sites: the Owens River Valley in Inyo County (L. Greene, California Department of Fish and Wildlife, written commun., 2015) and the upper San Luis Rey River, including part of the Cleveland National Forest in San Diego County (S.L. Howell, U.S. Geological Survey, unpub. data, 2019). Outside of these sites, flycatchers occur as small, isolated populations of one to six pairs (Kus and others, 2003).

Male flycatchers typically arrive in southern California at the end of April, whereas females arrive approximately 1 week later. While on the breeding grounds, males sing repeatedly from exposed perches. Once the pair bond is established, the female builds an open-cup nest that usually is placed in a branch fork of a willow (Salix spp.) or plant with a similar branching structure approximately 1–3 m above the ground. The typical clutch of three to four eggs is laid in May–June. Females incubate for approximately 12 days and nestlings fledge within 12–15 days, in early July. Adults usually depart from their breeding territory in mid-August/early September to their wintering grounds in central Mexico and northern South America (U.S. Fish and Wildlife Service, 2002).

The San Luis Rey Flood Risk Management Project Area (Project Area) spans approximately 233 hectares (ha; 576 acres) of the lower San Luis Rey River in northwestern San Diego County, California (fig. 1; table 1). Authorized in 1970 and constructed during the late 1980s and early 1990s, the flood control Project Area includes single- and double-levee reaches and six off-channel detention ponds, five of which also serve as mitigation sites for impacts to biological resources within the channel. Operation and maintenance of the flood control project includes periodic vegetation clearing, exotic plant removal, and sediment removal to ensure that sufficient water conveyance capacity is maintained (U.S. Fish and Wildlife Service, 2006b). Management of the off-channel ponds involves adaptive habitat management such as vegetation clearing, exotic plant removal, and planting native vegetation.

Riparian vegetation communities at the Project Area include willow-dominated riparian, mixed mule fat (Baccharis salicifolia) and sandbar willow (Salix exigua) riparian scrub, freshwater marsh, and areas dominated by non-native giant reed. Dominant plants include red/arroyo willow (Salix laevigata/ Salix lasiolepis; we did not distinguish between these species), black willow (Salix gooddingii), Fremont cottonwood (Populus fremontii), sandbar willow, mule fat, and giant reed. Neighboring habitat and land-use types include coastal sage scrub, non-native grassland, and urban housing and commercial developments. Human disturbances such as homeless camps, recreation, illegal dumping, introduction of invasive exotic plants and feral animals, and use by pets from neighboring houses are pervasive throughout the Project Area (table 2).

The Project Area includes a channelized 10.7-kilometer (km; 6.6-mile) section of the lower San Luis Rey River from Interstate 5 to College Boulevard and six detention ponds outside of the channel in Oceanside, California (table 1). The Project Area is divided into 12 survey sites, 5 of which are primarily within the flood control channel (hereafter “Channel”) and 7 that are outside of the channel (hereafter “Off-channel”; table 1). The seven Off-channel sites include one historic restored site north of the levee and west of Whelan Lake and six in the detention ponds. There are two large restoration sites, one in Reach 1 (Benet Restoration) and one in Whelan Mitigation (Whelan Restoration). One smaller restoration site is located in Reach 4 (College Restoration).

Before the 2006 vireo breeding season, the U.S. Army Corps of Engineers began two flood risk management activities for the San Luis Rey Flood Risk Management Project: (1) exotic species eradication and (2) flood risk management of river channel vegetation. Exotic species eradication primarily included removal and control of giant reed, but other exotic species such as pepperweed (Lepidium latifolium), tamarisk (Tamarix ramosissima), and other non-native tree species also were targeted for removal. Exotic species were physically removed or treated with herbicide. The purpose of the flood risk management was to remove vegetation from the San Luis Rey flood control channel to provide a level of flood conveyance of 2,016 cubic meter per second (m³/s; equivalent to a 150-year flood event; U.S. Fish and Wildlife Service, 2006b). In addition, a one-time mowing and mulching task was done in 2005 to reduce the risk of flooding during the 2006 rainy season. Ongoing flood risk management of river channel vegetation includes rotational and annual vegetation mowing in phases. Phase 1 includes vegetation clearing in Reaches 1–4 to achieve a minimum flow conveyance of 1,500 m³/s (equivalent to a 100-year flood event). Phase 2 includes mowing associated with maintaining the Phase 1 mowing area as well as restoration or creation of habitat for vireos and flycatchers. Phase 3 includes mowing associated with maintaining the Phase 1 mowing area as well as small sections of vegetation mowing in Reach 1 and Reach 3a. Rotational mowing is done every 5 years, alternating between two 18–23-m-wide strips of vegetation (Rotations 1 and 2) such that each strip is mowed every 10 years. For a detailed description of the timeline and vegetation treatments and management, see appendix 1.

In fall 2019, annual Phase 1 and 2 mowing in reaches 1–4 was completed. In total, 31.4 ha was mowed (appendix 1).

There were three Restoration areas within the Project Area: an exotic plant removal program in Reach 1 (Benet Restoration) that began in October 2012 and two projects that started in March 2014 (appendix 2, figs. 2.1–2.4). The largest project, focused on providing flycatcher habitat, was on the north side of the river channel in the eastern section of the Whelan Mitigation site (Whelan Restoration). A rock levee was removed and graded to below ground level, allowing flooding from the main channel to mimic more natural river flows. A second, smaller project occurred on the south side of Reach 4 near the College Boulevard bridge (College Restoration). Restoration activities included planting, bi-monthly watering, and spraying weeds with herbicide through 2015. Since 2016, there has been no watering or herbicide treatment, but we continued to classify these areas as Restoration sites and evaluate them separately. In addition to the three Restoration sites, in spring 2014, several small areas of the river were planted with pole cuttings, and in 2017, two of the Off-channel ponds were planted as a part of adaptive habitat management. These areas were considered passive restoration and were not included with our Restoration sites or included in any of our analyses of Treatment Index.

The purpose of this study was to document the status of vireos and flycatchers at the Project Area and characterize habitat structure and composition within the Project Area (fig. 1). Specifically, our goals for vireos were to (1) determine the size and composition of the vireo population; (2) characterize habitat used by vireos; (3) band vireos and resight banded vireos to estimate vireo survivorship, site fidelity, and dispersal; and (4) assess the effects of vegetation removal on vireo reproductive success and productivity by monitoring established nest monitoring plots in the Channel and Off-channel sites. Our goals for flycatchers were to (1) determine the size and composition of the Willow Flycatcher population at the Project Area, (2) document and monitor nesting activities of resident flycatchers, and (3) band and resight all flycatchers to facilitate the estimation of flycatcher survivorship and movement. The purpose of the habitat component of the study was to (1) provide post-treatment data on the habitat composition and structure of the Project Area including areas that underwent vegetation removal in 2005, 2007–12 and 2014–15, and 2017–19 and (2) characterize habitat use by vireos in response to vegetation management at the Project Area. These data, when combined with data from other years, will inform natural resource managers about the status of these endangered species at the Project Area and guide modification of land use and management practices as appropriate to ensure the species’ persistence.

Vireo and flycatcher surveys have been done at the Project Area every year since 2006. In 2020, four protocol surveys were done between March 31 and July 20, 2020, and followed standard survey techniques developed and recommended by the California Least Bell’s Vireo Working Group (now known as the California Riparian Birds Working Group) and USFWS Least Bell’s Vireo survey guidelines (U.S. Fish and Wildlife Service, unpub. data, 2001). We supplemented these protocol surveys with weekly territory monitoring visits; therefore, most survey sites were visited more than 10 times throughout the breeding season, resulting in complete coverage of the Project Area. We used recorded callbacks as needed during surveys and monitoring to confirm absence of vireos in areas in which no birds were detected and to attract males to check leg bands. Vireo field work was done by Armand Amico, Trevin Braun, Rachel Guinea, Alexandra Houston, Scarlett Howell, Suellen Lynn, Shannon Mendia, Ryan Pottinger, and Ben Stubbs (all under USFWS permit TE-829554), with assistance provided by Jessica Medina, Max Moore, and Gloria Salas.

We completed four protocol flycatcher surveys of the Project Area between May 20 and July 20, 2020, completing one survey during each of the four survey periods (U.S. Fish and Wildlife Service, 2000, Sogge and others, 2010). Flycatcher field work was done by Armand Amico, Rachel Guinea, Alexandra Houston, Suellen Lynn, Shannon Mendia, and Ryan Pottinger (all under USFWS permit TE-829554) with assistance provided by Jessica Medina, Max Moore, and Gloria Salas.

For both species, observers moved slowly (1–2 km per hour [0.6–1.2 miles per hour]) through the riparian habitat while searching and listening for vireos or flycatchers. Observers walked along the north and south levees to survey the flood control channel. In wider stands, observers traversed the habitat to detect all birds throughout its extent. Surveys were done between dawn and early afternoon, depending on wind and weather conditions. For each bird encountered, investigators recorded age (adult or juvenile), sex, breeding status (paired, single, unknown, or transient), and whether 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 using ArcGIS Collector and Survey 123 on Samsung Galaxy S7 and S8 and LG G5 mobile phones with Android operating systems and built-in Global Positioning System (GPS) to determine geographic coordinates (World Geographic System of 1984, WGS 84). Distance to the nearest surface water was recorded for each flycatcher location. Dominant native and exotic plants were recorded within each vireo and flycatcher territory, and percent cover of native vegetation was estimated using cover categories of less than 5 percent, 5–50 percent, 51–95 percent, and greater than 95 percent. Overall habitat type was specified according to the following categories:

We monitored vireo nests to evaluate the effects of native and exotic vegetation removal on nest success and productivity. Nest monitoring was done from March 30 until the last nest fledged on August 4, 2020. We performed work at three general site types: (1) within the flood channel where exotic and native vegetation removal has occurred regularly (Channel); (2) at three sites next to the flood channel (Upper Pond, Park Pond, and Whelan Mitigation; Off-channel); and (3) at three sites that have been restored (Restoration), two of which were within the Channel (Benet Restoration in Reach 1 and College Restoration in Reach 4) and one situated Off-channel (Whelan Restoration within Whelan Mitigation).

Territory boundaries were delineated by biologists in the field by circumscribing vireos’ nesting, singing, and foraging locations. In 2017, we began excluding areas in a vireo’s territory that were obviously avoided (for example, the vireo used the edges of the river channel but avoided the center) by creating two unconnected polygons, as opposed to a single large one spanning the area. We classified territories quantitatively by site type in ArcGIS (version 10.5, ESRI, 2016) using territory boundaries and the boundaries of the three site types (Channel, Off-channel, and Restoration). If a territory straddled more than one site type, we assigned it to the site type that comprised more than 50 percent of the territory. Sixty-three Channel vireo pairs and 1 single male, 28 Off-channel pairs and 3 single males, and 5 Restoration pairs were monitored during the 2020 breeding season.

Pairs were observed for evidence of nesting, and their nests were located. We visited nests as infrequently as possible to minimize the chances of leading predators or Brown-headed Cowbirds to nests; typically, there were three to four visits per nest. The first visit was timed to determine the number of eggs laid, the next visits to determine hatching and age of young, and the last to band nestlings (see next section). We removed cowbird eggs from nests depending on when they were found. In nests with fewer than three vireo eggs, cowbird eggs were removed no sooner than the 7th day of incubation to minimize the possibility of nest abandonment in response to the removal. We removed cowbird eggs from nests containing 3 or more vireo eggs as they were found. Cowbird nestlings were removed immediately from nests. Fledging was determined through direct observation of fledglings in the territory or, in some rare cases, inferred from an accumulation of feather dust and fecal material in the nest, indicative of vireo fledging. We recorded characteristics of nests, including nest height, host plant species, host height, distance to edge of host plant, and distance to edge of host clump following abandonment or fledging of nests.

The primary goals of banding vireos at the San Luis Rey Flood Risk Management Project Area were (1) to assess vireo site fidelity in response to vegetation management, (2) to examine natal dispersal within and outside of the Project Area in response to vegetation management, and (3) to understand how vegetation removal and alteration affected vireo demography. We banded nestlings from monitored nests at 6–7 days of age with a single dark blue anodized numbered federal band on the left leg. We captured adult vireos in mist nets at all of the monitoring sites and banded them with a unique combination of colored plastic and anodized metal federal bands, including an anodized dark blue band to designate the San Luis Rey River as the bird’s site of origin. Adults previously banded as nestlings with a single numbered metal federal band (natal birds) were target netted to determine their identity, and their original band was supplemented with other bands to generate a unique color combination. These data will supplement banding data currently being gathered by the U.S. Geological Survey (USGS) and other investigators on nearby vireo populations on the upper San Luis Rey River and the Santa Margarita River on Marine Corps Base Camp Pendleton (MCBCP).

We used Pearson’s chi-square analysis and Fisher’s exact tests to determine if there were differences in nest success, hatching rates, and fledging rates between Channel, Off-channel, and Restoration pairs. We included a “Mixed” category for instances where (1) a territory was categorized as one site type, but the vireos placed at least one nest in another site type or (2) vireos nested in two site types within the same territory. For example, we had territories that were categorized as “Restoration” (50 percent or more of the total territory area fell within the boundaries of the restoration), but the vireos placed their nest(s) within the Channel, avoiding areas of active restoration. In total, two territories were considered “Mixed” in 2020. For analyses involving the nest as the unit of analysis, we used the location of the nest (for example, a territory categorized as Channel could have a nest in a Restoration site). For any analyses of reproductive success or productivity that involved a measure of success per pair or territory, we analyzed Mixed territories separately. 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 two-sample t-tests, Mann-Whitney U-tests (two groups), and Kruskal-Wallis tests (three or more groups) to determine if there were differences in nest site characteristics between successful and unsuccessful nests within and between the Channel, Off-channel, and Restoration sites. T-tests were used when distributions were normal and variances were similar; Mann-Whitney U-tests or Kruskal-Wallis tests were used when the data violated these assumptions. We used analysis of variance and Tukey’s post-hoc pairwise comparisons to determine if there were differences between Channel, Off-channel, and Restoration sites in average clutch size and average number of young per pair in 2020 and average clutch size and number of young fledged per pair by year (2006–20).

We created an index of treatment (Treatment Index) to evaluate the cumulative effects of vegetation removal over time for 2019 and 2020 vireo territories. We restricted treatment type for this index to the vegetation removal that occurred from 2005, 2007–12, 2014–15 or 2017–19 in which vegetation (native or exotic) was cleared to the ground. We did not include any restoration or herbicide treatments because these treatments were distinct from vegetation removal and analysis of them was beyond the scope of this project. We limited our analysis of Treatment Index to territories in the Channel because Off-channel adaptive habitat management activities mitigated the impact of the vegetation removal, making the Treatment Index for those territories unusable for our analyses. We calculated the percent of each territory area that was treated in each year (2005, 2007–12, 2014–15, and 2017–19) by overlaying the 2019 and 2020 territory boundaries with the treatment areas in ArcGIS. Territory boundaries were created to include the bird location points collected using GPS throughout the field season. Treatment boundaries were generated from the location data collected using GPS in the field by a RECON Environmental, Inc. (https://www.recon-us.com/) biologist who walked the entire treatment boundary during vegetation removal activities. Because treatments have been occurring since 2005, it was possible to have multiple years of treatments as well as overlap among treatments within a single territory. Because early successional vegetation can recover quickly following disturbance, we assumed that the influence of the vegetation removal diminished over time; therefore, when there was overlap in treated areas among years, we used the most recent treatment year to calculate the Treatment Index (fig. 2). To calculate the Treatment Index, we used the percent of the territory area treated weighted by the squared inverse of the time (t²) since treatment summed across all treatment years. We chose t², rather than t because we wanted to capture the effect of the treatment diminishing quickly over time. We used the following formula: Treatment Index= ∑ t=2005 2019 percent Territory Area Treated t ×1/ n t 2 (1)where

t is the year of treatment,

To calculate the Treatment Index, calculate the percent area that intersects with the territory for (1) 2005 treatment and (2) overlap in 2005 and 2008 treatments.

We performed three sets of analyses using the Treatment Index. For all analyses of the Treatment Index, we excluded territories that overlapped with areas of active restoration.

We used RMark to model the effects of vegetation treatment on daily survival rate (DSR) of vireo nests. Calculation of nest success and the factors influencing it are critical components of studies of the breeding ecology of birds, particularly special status species. We calculated the DSR (the probability that a nest survived from one day to the next; Dinsmore and others, 2002). We used RMark to run program MARK to calculate DSR, which accounts for the variability in detection of nests at the various stages and allows for the analysis of the effects of covariates on DSR. In this study, we evaluated the effects on nesting success in two ways:

We looked at whether Treatment Index (for Channel territories only) had an impact on DSR in 2020. We also looked at whether habitat structure at the nest (2006–20) was correlated with DSR by analyzing the relationship between total vegetation cover at each height category (see “Vegetation Study Design” section) and DSR.

We calculated nest survival across a 30-day cycle (4 days egg-laying, 14 days incubation, 12 days nestling period) in which incubation begins with the penultimate egg. We calculated age of nests at the time they were discovered by forward- or backward-dating of nests in relation to known dates of nest building, laying, or hatching. We used an information-theoretic approach (Burnham and Anderson, 2002) to evaluate support for statistical models reflecting a priori hypotheses regarding the effect of treatment and habitat variables on DSR. We hypothesized that DSR would be inversely related to the Treatment Index and would increase with increasing foliage cover at nest sites, particularly understory cover within 2 m (6.6 feet) of the ground, where vireos place their nests. 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 and habitat variables on DSR. We then modeled the treatment and habitat covariates individually and evaluated support for each model in relation to the constant survival model and each other.

We sampled vegetation along permanent linear transects within three of the vireo monitoring sites (Channel and two Off-channel sites: Upper Pond and Whelan Restoration). Sampling points consisted of 2- by 2-m quadrats at 10-m intervals along each transect; the number of points sampled varied with the length of each transect (fig. 3). Transects were originally established in 2006 by using a systematic sampling design. To capture the range of variability of riparian vegetation structure and composition, we positioned transects perpendicular to the river channel. To provide uniform coverage, we placed transects at fixed distances from each other; distances varied with the size of the site. In the Channel, transects were placed at 200- or 400-m intervals depending on the width of the river. In Upper Pond, transects were placed every 100 m. In addition, we sampled two 350-m transects at Whelan Mitigation (Whelan Restoration), that were initially surveyed from 1991 to 1993, to monitor riparian restoration by the U.S. Army Corps of Engineers (Kus, 1998). The Whelan Restoration transects were 75 m apart and oriented approximately parallel (320 degrees) to the flood control channel. Before 2014, this site was sampled and used as an Off-channel site in analyses. However, in 2014, it became a Restoration site and has been analyzed separately from the Channel and the Upper Pond sites since then.

We used a number of permanent and semi-permanent methods to ensure that transects could be resampled in future years. First, a 1.5-m metal rebar was driven into the ground, leaving 75 centimeters (cm) above ground to mark the start of each transect. We spray-painted the rebar pink or orange and placed them at the intersection of the south levee and the riverbed. From the rebar, using a compass and tape measure, two field personnel measured the distances between sampling points. A numbered, wooden stake, spray-painted pink or orange, was placed in the ground and colored plastic flagging was tied nearby to aid in locating the points. Finally, we obtained geographic coordinates for each rebar and wooden stake using a GPS unit (Garmin GPS 12).

In this section, we present the results from surveys and nest monitoring on Least Bell’s Vireo population size and demographic parameters.

A total of 46 transects (526 sampling points) were sampled in the Project Area in 2020 (table 27; appendix 8, figs. 8.1–8.4). Seventy-one percent (376/526) of points were in the Channel and 22 percent (115/526) were located Off-channel in Upper Pond. The remaining 7 percent (35/526) were in the Whelan Restoration site. The number of points per transect varied between 4 and 18. Three points were in homeless camps and were not sampled in 2020. Global Positioning System coordinates for the start and end point of each transect are provided in appendix 9.

No breeding pairs of flycatchers were detected in 2020. This was the 14th year that the Project Area has not been occupied by Willow Flycatchers since it was colonized and monitoring began in 2000 (Kus and Rourke, U.S. Geological Survey, unpub. data, 2005). It is unknown why flycatchers did not breed in the Project Area, although conditions in the historical flycatcher territories have been gradually changing since 2006 when three pairs occupied the Project Area. At Whelan Restoration in particular, large canopy trees such as red/arroyo willows and black willows have been dying, and restoration efforts are still in early phases, so this historic breeding site is likely still too sparse and open for flycatchers.

Vegetation clearing does not appear to have played a role in the decline of the flycatcher in the San Luis Rey Flood Risk Management Project Area population. More likely explanations, in addition to habitat change, include other life history factors, such as juvenile or adult mortality experienced during migration or on the wintering grounds. At nearby Marine Corps Base Camp Pendleton, the resident population of flycatchers also declined for 10 years, from 26 individuals in 2007 to 0 individuals in 2017 (S. Howell, U.S. Geological Survey, unpub. data, 2017). In 2018, a social attraction study commenced at Camp Pendleton. In 2018–19, there were three resident breeders, and two resident breeders were documented in 2020 (S. Howell, U.S. Geological Survey, unpub. data, 2018, 2019, 2020). Collection of more information is warranted for this declining species.