Introduction

Grasslands in the midcontinent of North America have been lost or degraded by numerous factors, including agricultural conversion, energy development and other human activities, and invasion by woody vegetation and exotic species, leading to long-term declines in the amount of grassland on the landscape (Bernath-Plaisted and Koper, 2016; Carbutt and others, 2017; Scholtz and Twidwell, 2022). More than 80 percent of the historical extent of grasslands in this region has been converted to other uses since Euro-American settlement (Samson and Knopf, 1994; Niemuth and others, 2022). Remaining grasslands support a large suite of avian species that are undergoing long-term population declines. In North America, grassland birds as a group are declining more than birds that use other habitats (for example, forests, wetlands, and so forth) (Rosenberg and others, 2019). Consequently, an array of State, Provincial, and Federal agencies; non-governmental conservation organizations; and private entities in the North American midcontinent have prioritized the conservation and management of grasslands and their associated bird communities by enacting conservation and management programs that aim to sustain wildlife populations and the habitats they use (Askins and others, 2007; Bernath-Plaisted and others, 2023).

Grassland habitats are periodically managed with disturbance (for example, grazing, haying, and burning) to maintain desired grassland species composition and structure, control noxious weeds and woody vegetation, and provide forage for livestock (Herkert and others, 1996; Askins and others, 2007). However, the annual timing of these practices has the potential to disrupt the nesting activities of grassland birds, which is an especially important concern for birds nesting on conservation program lands. Throughout the midcontinent, land-management agencies strive to use the best available information to time grassland management actions to avoid adversely affecting birds nesting in these habitats (for example, through nest abandonment, destruction of nests, or mortality of chicks or adults). Consequently, information about when grassland birds are actively nesting is needed to inform decisions about the timing of these management actions. Species-specific differences in life-history strategies, abundance or rarity, location and extent of breeding ranges, and other factors have resulted in nesting phenologies that vary greatly among different species of grassland-nesting birds and in different regions of the midcontinent (Johnson and others, 2019). To our knowledge, this phenological information has not been compiled throughout this entire region in a manner that will be useful to wildlife and grassland resource managers.

The purpose of this report is to compile information on the seasonal occurrence (that is, the duration of presence during the annual cycle) and nesting phenology (that is, the time from the first egg laid to the last young fledged from a nest in a region) for 38 species of nonwaterfowl, grassland-nesting birds across a broad swath of the North American grassland biome. We summarize when each species is present in a location, as well as when each species generally nests. The information is organized by taxonomic groups and summarized for broad regions of the midcontinent of North America. Although this published information was sometimes incomplete at the individual State or Provincial level, we provide available information for each focal State and Province in appendix 1. This summarized information can be used by managers who make decisions that balance the health of grassland habitats with the needs of agricultural producers and the birds nesting in these habitats.

Methods

Study Area and Focal Species

The study area encompasses seven Bird Conservation Regions associated with grassland and grassland-sage ecosystems in the midcontinent of North America (fig. 1; North American Bird Conservation Initiative, 2014). This expansive region intersects 18 States and 3 Provinces and represents the approximate extent of the short-, mixed-, and tallgrass prairie ecosystems of North America. This region was historically dominated by grassland land cover, but currently most of it has been converted for agricultural land uses, especially in grasslands in the eastern portion of this region (Samson and Knopf, 1994; Niemuth and others, 2022).

We selected a suite of 38 nonwaterfowl bird species representing five taxonomic groups (that is, upland gamebirds, shorebirds, herons, raptors and owls, and passerines) that nest in grassland habitats in the midcontinent (table 1); waterfowl were excluded because they are being addressed in a companion study. The list of species is representative of the broad range of grassland habitats present in the midcontinent of North America, and it was informed by the species included in Johnson and others (2019), with a few modifications. The list includes obligate grassland birds (that is, species that are exclusively adapted to and entirely dependent on grassland habitats) and facultative grassland birds (that is, species that use grasslands as part of a wider array of habitats) (Vickery and others, 1999). Although the focus in this report is to inform habitat management decisions for a diverse assemblage of grassland bird species, there is considerable variation in the population status and conservation attention among the focal species (Fields, 2017; Sauer and others, 2020). Additionally, these species vary widely in life-history characteristics that could affect the phenology of their nesting (Winter and others, 2004; Jones and others, 2010), such as affinities for specific habitat structures (Perlut and others, 2006; Churchwell and others, 2008), precocial versus altricial chick development, migration distance or year-round residency, and the propensity to renest or attempt multiple broods (Lloyd and Martin, 2005).

Table 1.    

Alpha codes, common and scientific names, and taxonomic group membership of 38 focal grassland bird species nesting in the midcontinent of North America. Vernacular and scientific names follow Chesser and others (2023), and species are listed in taxonomic order.

Table 1.    Alpha codes, common and scientific names, and taxonomic group membership of 38 focal grassland bird species nesting in the midcontinent of North America. Vernacular and scientific names follow Chesser and others (2023), and species are listed in taxonomic order.

Alpha code	Common name	Scientific name
Upland gamebirds
NOBO	Northern bobwhite	Colinus virginianus
GRSG	Greater sage-grouse	Centrocercus urophasianus
STGR	Sharp-tailed grouse	Tympanuchus phasianellus
GRPC	Greater prairie-chicken	Tympanuchus cupido
LEPC	Lesser prairie-chicken	Tympanuchus pallidicinctus
GRAP	Gray partridge	Perdix perdix
RNEP	Ring-necked pheasant	Phasianus colchicus
Shorebirds
MOPL	Mountain plover	Charadrius montanus
UPSA	Upland sandpiper	Bartramia longicauda
LBCU	Long-billed curlew	Numenius americanus
MAGO	Marbled godwit	Limosa fedoa
WILL	Willet	Tringa semipalmata
WIPH	Wilson's phalarope	Phalaropus tricolor
Herons
AMBI	American bittern	Botaurus lentiginosus
Raptors and owls
NOHA	Northern harrier	Circus hudsonius
SEOW	Short-eared owl	Asio flammeus
Passerines
HOLA	Horned lark	Eremophila alpestris
SEWR	Sedge wren	Cistothorus stellaris
SPPI	Sprague's pipit	Anthus spragueii
CCLO	Chestnut-collared longspur	Calcarius ornatus
TBLO	Thick-billed longspur	Rhynchophanes mccownii
GRSP	Grasshopper sparrow	Ammodramus savannarum
LASP	Lark sparrow	Chondestes grammacus
LARB	Lark bunting	Calamospiza melanocorys
CCSP	Clay-colored sparrow	Spizella pallida
FISP	Field sparrow	Spizella pusilla
BRSP	Brewer's sparrow	Spizella breweri
VESP	Vesper sparrow	Pooecetes gramineus
LCSP	LeConte's sparrow	Ammospiza leconteii
NESP	Nelson's sparrow	Ammospiza nelsoni
BAIS	Baird's sparrow	Centronyx bairdii
HESP	Henslow's sparrow	Centronyx henslowii
SAVS	Savannah sparrow	Passerculus sandwichensis
BOBO	Bobolink	Dolichonyx oryzivorus
CHME	Chihuahuan meadowlark	Sturnella lilianae
EAME	Eastern meadowlark	Sturnella magna
WEME	Western meadowlark	Sturnella neglecta
DICK	Dickcissel	Spiza americana

Review of Phenology Information

We searched more than 50 sources of compiled avian phenology information (that is, published books and breeding bird atlases containing information on avian distribution and occurrence; appendix 2), as well as the eBird database, an online citizen-science database of global bird observations (Sullivan and others, 2009). From those sources, the following information was compiled for each of the 38 focal species in each of four author-defined regions in the midcontinent of North America (fig. 2): (1) species- and region-specific arrival and departure dates from the eBird database (that is, “seasonal occurrence”), and (2) species- and region-specific reported start and end dates of nesting activity and the duration of the nesting period (that is, “nesting phenology”).

For seasonal occurrence information, we accessed line graphs of frequency from the eBird database (eBird, 2023; that is, the frequency with which a species was reported to eBird within a region, measured by the percent of eBird checklists reporting the species during each week of the year) for each of the 38 species in our regions of interest. We considered eBird data from the past 50 years (that is, 1973–2023), because these five decades approximated the period of most of the nesting phenology information. We visually assessed and disregarded extreme outliers in the data. For migratory species, we assessed the graphs and recorded the approximate date (that is, the week) that a species arrived on its breeding grounds and the week that a species departed its breeding grounds for each region. A species was recorded as being a year-round resident (that is, nonmigratory) in a region if the eBird frequency graph did not show distinct periods of arrival and departure. An example for a migratory species, the Baird’s sparrow [Centronyx bairdii], in North Dakota, is shown in figure 3. In this example, the species’ arrival and departure dates in North Dakota, respectively, were recorded as May 1 and August 31.

For nesting phenology information, we reviewed available books and breeding bird atlases, including some sources that were available only online, for the 18 States and 3 Provinces in the study area. Books and atlases were searched for information on nesting phenology; such information often included extreme nesting dates (beginning and end dates), egg dates (dates when nests with eggs have been known to occur), as well as qualitative statements by the author(s) about when the species is likely to be nesting based on the authors’ experiences or based on unknown sources. Occasionally, publications summarized rigorously collected information from a breeding bird atlas (for example, they provided histograms of nesting dates for nests that were reported by atlas observers). All information available in these books and atlases was compiled. In some cases, exact dates were provided (for example, “May 16 to July 14”). Other times, the information was less precise (for example, “mid-May to mid-July”) or the information was presented in a histogram or other graphic presentation. In the latter situation, we summarized the information using common decision rules to aggregate information at the level of the week (for example, “first week of May” became May 1, “mid-May” became May 15, and “third week of May” became May 22). Sources of phenology information were based on the entire State or Province, rather than just the grassland portions illustrated by the study area boundary in figure 2.

Overall, published nesting phenology information was located for 19 of the 21 States and Provinces in the study area: Alberta, Saskatchewan, Manitoba, Montana, North Dakota, South Dakota, Minnesota, Colorado, Nebraska, Kansas, Iowa, Missouri, Wisconsin, Illinois, Indiana, Ohio, Michigan, Oklahoma, and Texas. We lacked nesting phenology information for New Mexico and Wyoming. In the case of New Mexico, the book written on the State’s birdlife (Bailey, 1928) contained no nesting phenology information, and a breeding bird atlas from the early 2000s was not completed (Dave Krueper, U.S. Fish and Wildlife Service, retired, written commun., 2023). In the case of Wyoming, two books have been written about the State’s birdlife (Knight, 1902; Faulkner, 2010), but neither contained nesting phenology information. A formal breeding bird atlas also has not been published for Wyoming.

Because not all States and Provinces had nesting phenology information, we aggregated information into four larger regions that were defined based on similarities in latitude, comparable species-specific arrival and departure dates from the eBird database, and personal knowledge of avian phenology in each location. The four regions were defined as Canadian grasslands (Alberta, Saskatchewan, and Manitoba), northern U.S. grasslands (Montana, North Dakota, South Dakota, Wyoming, and Minnesota), central U.S. grasslands (Nebraska, Colorado, Kansas, Iowa, Missouri, Wisconsin, Illinois, Michigan, Indiana, and Ohio), and southern U.S. grasslands (New Mexico, Oklahoma, and Texas) (fig. 2). We compiled existing phenology information for the constituent States or Provinces in a region and calculated the median dates for the start and end of nesting, which are the dates reported in the figures.

Results and Discussion

In this report, two types of phenology information (that is, seasonal occurrence and nesting phenology) are summarized for four regions consisting of 18 States and 3 Provinces in the North American midcontinent (figs. 4–7; table 2). The phenology information available at the level of the individual State or Province is also summarized and is provided in appendix 1 (figs. 1.1–1.21). These summaries provide information about relative differences in timing and duration of a species’ occurrence in a region as well as when a species is likely to be nesting. This information will help resource managers better understand when grassland species are likely to be present and nesting in the grasslands they manage throughout the midcontinent.

The published nesting dates and nesting durations for the 38 species of nonwaterfowl grassland birds in the four regions in the midcontinent are summarized in table 2. For all species combined in a region, the total nesting durations were 149 days (March 24 to August 19) for Canadian grasslands, 173 days (March 25 to September 13) for northern U.S. grasslands, 189 days (March 13 to September 17) for central U.S. grasslands, and 164 days (March 2 to August 12 for southern U.S. grasslands. These dates extend beyond and comprise longer durations than those of the primary nesting seasons set by the U.S. Department of Agriculture to avoid disturbance to nesting cover and nesting birds in grasslands enrolled in the Conservation Reserve Program (Farm Service Agency, 2020), but it is important to note that the current primary nesting season dates for individual States are based on the species selected by the State technical committees of the U.S. Department of Agriculture. Nesting dates reported here, however, can be interpreted as likely minimum and maximum nesting dates. Information is currently lacking to estimate the proportion of active nests throughout the breeding season, which could help managers evaluate tradeoffs in timing of grassland managements actions.

Compiling this information provided a unique opportunity to examine patterns in nesting phenology across species and guilds. In general, native upland gamebirds (northern bobwhite [Colinus virginianus], sharp-tailed grouse [Tympanuchus phasianellus], greater prairie-chicken [Tympanuchus cupido], lesser prairie-chicken [Tympanuchus pallidicinctus]) tended to begin nesting later and (or) had shorter nesting durations in northern areas compared to southern areas. The ring-necked pheasant (Phasianus colchicus), a non-native upland gamebird, begins nesting in mid-April throughout its range in the United States but shows an opposite pattern to its native counterparts, with a longer nesting duration in northern areas than in southern areas. Most shorebirds show a different pattern from those exhibited by native upland gamebirds. Upland sandpiper (Bartramia longicauda), marbled godwit (Limosa fedoa), willet (Tringa semipalmata), and Wilson’s phalarope (Phalaropus tricolor) tended to begin nesting earlier and (or) had longer nesting durations in northern areas than in southern areas. There were few noteworthy patterns for passerines. The eastern meadowlark (Sturnella magna) and dickcissel (Spiza americana) tended to begin nesting later and (or) had shorter nesting durations in northern areas compared to southern areas, which is similar to the pattern shown for native upland gamebirds. For some passerine species (for example, vesper sparrow [Pooecetes gramineus], Savannah sparrow [Passerculus sandwichensis], and bobolink [Dolichonyx oryzivorus]), it was noteworthy that the beginning of their nesting seasons and their nesting durations were consistent in all regions in which the species were present (table 2). Patterns between or among species in this report could be coupled with results from Anteau and others (2023) to help managers make species- or guild-oriented management decisions.

The amount and type of nesting phenology information from the above publications varied tremendously. For example, some authors summarized information rigorously collected by citizen scientists and researchers as part of an official breeding bird atlas (for example, Kingery, 1998). Such information often was presented in the form of histograms of nesting activity. Also commonly reported were start and end dates of nesting or a list of dates when nests or nests with eggs were discovered (for example, Stewart, 1975). Even when such information was reported, the number of nests present for each species with phenology information was often limited (less than 30 nests in a single year or during 2–3 years). However, most of the nesting phenology information was coarser, consisting of qualitative statements by the author(s) that reflected expert opinion or unknown sources.

Figures presented in this report that summarize phenology information should be interpreted and used with caution for several reasons. First, the amount and level of detail on phenology provided in the published literature varied among sources. The data in some sources were collected with rigor (for example, breeding bird atlases), whereas data in other sources represented estimates from unknown sources or opinions of the expert(s) that authored the species account(s). Expert estimates or opinions can be affected by information available at the time of publication or the individual experiences of the author(s), which could be subject to bias. Second, some of the published information in books and breeding bird atlases was collected or published decades ago, and the breeding phenology of certain species of North American birds may have changed over time with land use and land cover change, climate change (Knudsen and others, 2011), and shifts in breeding ranges (Langham and others, 2015). Third, apparent trends in spring arrival or fall departures dates, start or end of nesting, and nesting duration may be artifacts of the information that was reported in the books and atlases (that is, the lack of data in some locations and the varying level of precision in how dates were reported) rather than representative of precise biological patterns. This is true even for the seasonal occurrence data from eBird. Fourth, the published nesting phenology information includes only those dates a nest was determined to be active and may not be wholly representative of a species’ entire nesting season or when young may be vulnerable outside of a nest. And finally, phenology can vary considerably within or among a region, State, or Province based on local climatic conditions, elevation, and many other factors (Anteau and others, 2023).

This project revealed notable gaps in the understanding of avian phenology in the grasslands of the midcontinent of North America and the need for better reporting and data availability of nest phenology information by researchers. Given that much of the available nesting phenology information was published decades ago and that the timing of nesting and migration may have changed for some species (for example, Bates and others, 2023), updating this information at State and Provincial levels would be prudent, especially considering a changing climate (Langham and others, 2015). Additionally, the available nesting information defined nesting as ending when young leave the nest bowl, but that definition misses a critical window of vulnerability for many bird species in which the young are still dependent on parental care and may have home ranges that extend well beyond the adults’ breeding territory. For example, Suedkamp Wells (2005) reported that postfledging resource use differed from adult resource use for dickcissels and eastern meadowlarks. There is growing interest in developing realistic guidelines for grassland management that incorporate adult and juvenile life-history needs on public and private lands (for example, Suedkamp Wells and others, 2008; Wolcott and others, 2023). The lack of information in the literature on the duration of the postfledging period, as well as minimum area requirements of fledglings for most grassland bird species, is an important gap that should be considered when making decisions about the timing of management actions in grasslands.

Despite the above limitations, this report unifies and summarizes disparate sources of nesting phenology information for many grassland-nesting bird species of the North American midcontinent for the first time and provides resource managers with a single source of the best currently available information (as of 2023) about the timing and duration of the nesting period. We encourage grassland bird researchers to collect and make available nesting data that would improve the understanding of nesting phenology for grassland birds in the midcontinent.