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Article

Urban Environmental Predictors of Group Size in Cliff Swallows (Petrochelidon pyrrhonota): A Test Using Community-Science Data

by
Cassie Rueda
1,2,3 and
Kevin J. McGraw
1,4,*
1
School of Life Sciences, Arizona State University, Tempe, AZ 85827, USA
2
Barrett the Honors College, Arizona State University, Tempe, AZ 85287, USA
3
Department of Sociology, Washington State University, Pullman, WA 99164, USA
4
Department of Integrative Biology, Michigan State University, East Lansing, MI 48824, USA
*
Author to whom correspondence should be addressed.
Birds 2025, 6(2), 17; https://doi.org/10.3390/birds6020017
Submission received: 17 February 2025 / Revised: 26 March 2025 / Accepted: 28 March 2025 / Published: 3 April 2025
(This article belongs to the Special Issue Resilience of Birds in Changing Environments)
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Simple Summary

We used a five-year community-science database to examine urban environmental predictors of group size in Cliff Swallows (Petrochelidon pyrrhonota) around Phoenix, Arizona, USA. We found that swallows formed larger groups in areas closer to water bodies and to cropland, suggesting that proximity to food and water resources support larger aggregations of these colonial birds in a highly and recently human-disturbed desert landscape.

Abstract

Due to continuing worldwide urban expansion, research into how urban environments affect local flora/fauna has grown significantly. Studies on the impacts of urbanization on birds have explored a wide variety of behaviors (e.g., foraging, breeding, migratory), but there is little research on the impacts of cities on avian coloniality. Various urban-environmental factors may impact colonial birds. The predominance of impervious surfaces in cities, for example, has been associated with the decline of several bird species due to negative effects on availability and quality of habitat. The urban heat island effect and shifts in resource availability (e.g., food, water) may also affect colonial birds. Here, we used five years of community-science data available in eBird to investigate urban impacts on group size in Cliff Swallows (Petrochelidon pyrrhonota), an abundant colonial bird species that now breeds readily under bridges and other built structures over or near water in Phoenix, Arizona, USA. We hypothesized that, based on the colonial breeding habits of these neotropical migratory birds in this desert environment, swallows in Phoenix would form larger groups in areas with more food and water sources and with more built structures. In fact, we found that proximity to water sources and cropland, but not impervious surface density, was positively and significantly related to group size. These results suggest that, in this desert ecosystem, an abundance of food/water resources provided by humans permits Cliff Swallows to form larger social groups during breeding. Although many studies show harmful impacts of cities on local wildlife, our findings highlight how urban and/or agricultural ‘oases’ may relieve some native species from natural resource limitations and permit them to thrive and increase in group size in human-impacted environments.

1. Introduction

More people live in cities currently than at any other time in human history, and as many as seven billion people are projected to reside in urban areas by 2050 [1]. This urban expansion has led to an increased interest among urban ecologists [2] in investigating the effect of human activities in built environments on local flora and fauna. Birds have been among the more commonly studied groups in relation to the impacts of cities on population distributions and phenotypic (e.g., behavioral, morphological) responses [3,4,5].
A wide range of behaviors has been studied in urban birds, including how urban environments influence foraging [6,7], breeding [8], and migration [9]. These studies have shown mixed effects of urbanization, from smaller clutch sizes when breeding and disorientation due to light pollution [8,9] to increases in habitat [10] and the use of gardens and backyards as additional food sources [7]. However, the impacts of cities on coloniality remain poorly known, especially among vertebrates. Although coloniality has long been studied in social insects, the effects of urbanization on their group dynamics appear to vary among species from negative, to neutral, to positive [11,12,13]. The study of coloniality in birds has often been restricted to the study of seabirds [14,15] and wetland birds [16]; by contrast, we stand to learn much more about colony dynamics in terrestrial birds [17], including in species that occupy agricultural (e.g., weaverbirds [18]) or urban (e.g., blackbirds, swallows [19]) environments [20,21].
There are several aspects of urban environments that may impact colonial species. Cities typically have large areas of impervious surface (e.g., pavement, concrete [22]), which replace natural habitats and thus may erode many aspects of daily life (e.g., feeding and breeding sites [23]). Increases in impervious surfaces have been associated with the decline of many bird populations [24,25,26], though to date all studies have been on territorially breeding, and not colonial, species. Urban areas also typically are warmer in ambient temperatures (e.g., urban heat island effect [27]), have altered water supplies [28], and harbor lower prey biodiversity (i.e., for insectivores/predators [29]). These factors are relevant to colonial birds because, in some species, the abundance of suitable foraging habitat within a colony’s maximum foraging range is related to colony size [30], and these alterations to the urban environment may negatively impact colony size. However, positive effects of cities on avian foraging and breeding have also been observed, such as an increase in breeding habitat for crevice-nesting birds [10], and an increase in the presence of ‘generalist’ feeding species [31]. Therefore, it is possible that certain terrestrial colonial birds may not see a significant decrease in abundance or group size in urban areas.
The goal of this study was to investigate urban environmental predictors of variation in group size in a widespread and abundant colonial vertebrate species, the Cliff Swallow (Petrochelidon pyrrhonota). Cliff Swallows are one of the most social terrestrial avian species in the world, forming colonies numbering well into the thousands of breeding pairs [30]. Their unique coloniality and wide distribution across North America have sparked interest in their ecology and behavior for many decades [32,33,34]. Each spring, Cliff Swallows migrate north from Central and South America for breeding, often returning to the same breeding locations [35,36]. As cities have grown across North America since the 1980s, Cliff Swallows have expanded their range of breeding habitats beyond cliffs to include building reusable mud nests beneath bridges and other man-made structures [37]. This adaptation can be seen through the history of recent studies aiming to prevent cliff swallow nesting on man-made structures [38,39,40]. Their presence in both urban and rural environments makes them an excellent candidate for a study on the impacts of urbanization on coloniality.
Although expanded urban environments appear to provide more potential habitats for Cliff Swallows, it is not clear if or how the degree of habitat urbanization in an area affects group dynamics in this colonial species. We gathered data on urban and environmental parameters in Maricopa County, Arizona, USA—the 10th largest metropolitan area in the United States [41] and one of the fastest growing counties in the country—and compared these to cliff swallow group-size data that we compiled from eBird [42,43,44], an open-source online citizen-science database of bird sightings that has been increasingly used by scientists to track avian abundance and distribution [45,46,47]. Group size (i.e., number of birds in a flock) was chosen for study, rather than breeding colony size per se, because eBird does not provide sighting information at the colony level. In addition to reporting number of birds seen, eBirders also often report information on location, date, and observer effort (e.g., number of observers, distance traveled, time spent), all of which can be used to rigorously test for environmental correlates of group size.
Based on the observations that cliff swallow breeding colonies have recently expanded into urban areas, which are home to many built structures for nesting (including roadway/canal bridges), as well as that these environments potentially contain fewer suitable foraging locations (due to the increased presence of impervious surfaces), we predicted that urban environmental factors like the percentage of impervious surface would be negatively associated with Cliff Swallow group size. However, based on the fact that desert cities like Phoenix now provide large, stable artificial water supplies (irrigation canals, lakes, ponds, etc.), which may be key for the insectivorous foraging habits of these birds over water [48], we also predicted that proximity to, or amount of, water would be positively associated with group size. A previous study over a 30-year period in rural Nebraska showed that the strongest predictor for colony size was the type of nesting substrate (e.g., bridges tended to have higher colony sizes than other colonies [49]). Thus, in light of the aforementioned potential for both positive and negative effects of urbanization on birds, it is also possible that group size in Cliff Swallows is positively impacted by urbanization, due to the abundance of reliable sources of urban standing water and the abundance of bridges for breeding habitat.

2. Methods

2.1. Acquisition of Swallow Group-Size Dataset

We downloaded eBird sighting data (ebird.org, accessed on 16 February 2025) for Cliff Swallows in Maricopa County during the months of March–June (their local breeding period) from 2018–2022. Maricopa County (https://en.wikipedia.org/wiki/Maricopa_County,_Arizona, accessed on 16 February 2025) contains over 4 million people (as of 2023) and 50 different cities spread over its vast land area (24,000 square km), which also includes many still-undeveloped natural areas, thus providing a rich opportunity to examine spatial and environmental variation in relation to bird distributions. After removing duplicates (i.e., separate rows containing identical information in every column except the User ID column), we determined the number of unique sites within each year, with the entry containing the highest observed group size at each unique site chosen to represent it. To determine what constituted a unique site, we applied a 100 m (radius) buffer, with intersecting sites merged based on which location contained more observations. We took this step to help prevent cases where a set of observations from slightly different locations were all observing a group at one location (e.g., multiple observations on different sides of a small pond). In cases where there was a tie for the highest observation count, a random location was chosen using a virtual die (with the number of sides on the die equal to the number of observations being chosen between). This site selection was conducted for each year of data before being combined into a single dataset containing 905 unique observations (Figure 1). For these observations, we saved the following eBird data: number of Cliff Swallows sighted, number of observers, observer distance traveled, time spent observing, latitude/longitude, and date (see Table S1 for additional data on swallow sightings).

2.2. Urban and Environmental Parameters

In addition to the above eBird variables, we considered the following urban and environmental predictors of cliff swallow group size: environmental variables (average monthly air temperature/precipitation, and proximity to major rivers, canals, and large lakes), human demographic variables (human population density and racial-minority percentage by census tract), and land-cover variables (proximity to open developed space, low/medium/high developed space, barren land, forests, grasslands, pastures, cropland, woody wetlands, and emergent herbaceous wetlands; Table 1). We obtained mean monthly precipitation and temperature data for each datapoint (i.e., on the date and at the site of observation) from the PRISM Climate Group (https://prism.oregonstate.edu; data accessed July 2022–January 2023) using the Extract Multi Values to Points function in ArcMap 10.7.1 [50] (ESRI 2019, ArcMap: Release 25. Redlands, CA, Environmental Systems Research Institute). We used the Near function in ArcMap to calculate distance from each swallow observation site to the nearest primary road and secondary road using the 2019 U.S. Census TIGER/Line databases (see definitions of road categories in https://catalog.data.gov/dataset/tiger-line-shapefile-2019-series-information-for-the-primary-and-secondary-roads-state-based-sh, accessed on 16 February 2025).
Additionally, we used the 2020 U.S. Census (by census tract) to determine human population density at each swallow observation site, as well as the percentage of minorities within the site’s tract. Racial demographics were included because racist practices in historical urban planning have directly led to contemporary ecological impacts, including luxury effects on animal biodiversity, within cities [51]. We used OpenStreetMap (OSM Water Layer is available for download at IIS U-Tokyo webpage: http://hydro.iis.u-tokyo.ac.jp/~yamadai/OSM_water/, accessed on 16 February 2025) to identify water sources and then used the Near function in ArcMap to extract the distance from each swallow observation site to each nearest water-source type (canal, major river, and large lake and river). We obtained land-cover and impervious-surface data using the USGS National Land Cover Database (2019 edition), again using the Near function in ArcMap, to calculate distance from each swallow observation site to each land-cover class: developed (open space), developed (low intensity), developed (medium intensity), developed (high intensity), barren land, forest, grassland, pasture, crops, woody wetlands, and emergent herbaceous wetlands. Additionally, we used the Focal Statistics tool in ArcMap to determine the percentage of the area surrounding an observation site (1 km radius, which is the general foraging range for Cliff Swallows [52]) that was of a specific land cover designation, as well as its average impervious-surface percentage. Once all data were collected, they were natural-log transformed if needed to meet normality assumptions for statistical testing (i.e., lnduration being the natural log of the time spent observing Cliff Swallows for a specific entry).

2.3. Data Reduction and Variable Selection

To account for potential multicollinearity among predictor variables, we assembled a correlation matrix for all five years of data (Figure 2). When two variables were significantly multicollinear (p < 0.05), and in the same direction, in four or all five of the years of data, we considered them redundant and proceeded to remove those parameters and leave only uncorrelated variables; these included: duration of eBird observation period (lnduration, in minutes; ln is used to represent the natural-log transformation), monthly mean temperature (temp, in °C), distance to nearest major river (lnDisMaR, in km), monthly mean precipitation (PPT, in mm), distance to nearest cropland (lnDisCro, in km), percent of a site’s census tract made up of minorities (percmin), percentage of surrounding area consisting of forest (FocalForest), and percentage of surrounding area consisting of impervious surfaces (FocalImp).

2.4. Statistics

All analyses were run using JMP Pro 16 (SAS Institute Inc., Cary, NC, USA). With the above-listed independent variables, and entering year and lnduration as random effects, we conducted an ordinary least-squares (OLS) regression analysis to determine significant predictors of cliff swallow group size. After running this global model, we determined a best-fit model, among all possible combinations of variables in the global model, using the lowest Akaike’s information criterion (AICc [53]), such that the best-fit model(s) is/are the one(s) whose AICc is five units less than that of the next-closest model.

3. Results

In the global model (Table 2), we found that observation duration as well as two environmental variables—distance to nearest river (lnDisMaR) and distance to nearest cropland (lnDisCro)—significantly predicted Cliff Swallow group size. In both cases, the relationship was negative, such that Cliff Swallow groups were larger in areas closer to a major river and to cropland (Figure 3). In the best-fit model (Table 3), three predictor variables were retained—distances to nearest river and nearest cropland, as well as % of impervious surface (FocalImp). However, only distances to nearest river and nearest cropland were statistically significant (Table 3). Although the % of impervious surface was not statistically significant (p = 0.07), it did not significantly improve or worsen the best-fit model to remove it (an AICc of 2885.4 with it, and 2886.6 without it).

4. Discussion

We examined the extent to which a broad set of urban and environmental factors explained variation in community-scientist-generated data on group size in a widespread and abundant colonial vertebrate bird species (the cliff swallow). We found that close proximity to a major river and to cropland significantly predicted larger group sizes in Cliff Swallows over a five-year period in and around a growing metropolitan area in the desert-southwestern United States (Phoenix, Arizona). To date, studies on group size in birds and mammals from urban environments have reached a variety of conclusions. Birds generally increase in population density in cities [54]. In spotted hyenas (Crocuta crocuta), social networks have been found to decrease in density when human activity increases [55], while among Floridian key deer (Odocoileus virginianus clavium), group sizes increased the closer they were to urban areas [56]. Past studies of Cliff Swallows have focused on colony-size variation in rural populations [57], but this study provides the first look into how urbanization impacts group-size variation in urban Cliff Swallows.
Interestingly, our findings differ from previous work on rural Cliff Swallows (from Nebraska), whereby increased levels of cropland were linked to lower colony sizes [52]. Thus, the relationship between cropland and group size in this species appears to be complex in both space and time, but to explain the differences it may be important to highlight the extreme environmental differences between our sites and those in past studies (as well as those across much of the rest of the breeding range of Cliff Swallows). In urban environments, generally, where impervious surfaces can limit water availability, these birds may have a more difficult time finding consistent sources of the insects they consume. However, as of 2018, up to 70% of Arizona farms were watered with flood irrigation [58], meaning cropland and its associated agricultural areas in Maricopa County may serve as regular sources of water and rich sites for Cliff Swallows to feed on insects. Thus, the arid but agriculture-rich environments around Maricopa Country in Arizona may help to explain why a strong positive relationship was found here between cropland and group size, but not in colony-level studies of Nebraskan Cliff Swallows [52].
In contrast, our findings did match previous results in this species on how water affects colony size, such that greater access to flowing or standing water was also associated with increased colony size of Cliff Swallows in rural Nebraska [52]. The strong relationship between group size and the proximity to a major river makes intuitive sense, as Cliff Swallows rely on sources of water for both nest construction (e.g., moist mud for building nests) and for feeding on flying insects over water [48]. This combined information on the importance of cropland and water proximity for increasing swallow densities could be valuable for urban planning and biodiversity-conservation strategies in an arid city like Phoenix, Arizona [59].
Although we found that the link between cliff swallow group size and the percentage of impervious surfaces in the surrounding area was only marginally statistically significant, this weak negative relationship is consistent with literature on the relationship between urban impervious surfaces and the decline of other bird species [24,25,26]. There are many reasons why more pavement and concrete in cities may decrease avian species richness or limit group size, including the urban heat island effect [60,61], as well as overall decreases in quantity [62] and quality [63] of food and water resources. One specific aspect of impervious surfaces that we were not able to explicitly quantify in our study, but is key to the lives of breeding Cliff Swallows, is the availability of nesting substrate beneath bridges/overhangs at or near the swallow observation sites. Had we been able to separate these surfaces from other built structures in the city, it is possible that we could have uncovered a link between swallow group size and structural surface-area for breeding. We encourage future work in this area, and with special emphasis on swallow colony size (i.e., number of nests/pairs at a site), not just the general group-size data we were able to acquire over wide space and time from eBird.
We recognize that studies utilizing eBird as a method of data collection and analysis are subject to a number of other limitations [42,64]. For example, data from open-source citizen science projects such as eBird are inherently subject to distributions of data that do not necessarily fully encompass the reality of a species’ full spatial and/or temporal distribution [65]. For example, not all locations of Cliff Swallows may be consistently accessible or visited by human observers, whether due to the restrictions of private/seasonal property or features of the landscape. Other areas may simply be less desirable for eBird users for a variety of reasons ranging from distance to the nearest residential center to exposure to the sun. Also, an inherent limitation of the usage of databases such as the 2019 National Land Cover Database is that it cannot be entirely accurate for years other than the specific year in which its data were collected, as land cover can change quickly [66]. This is difficult to address on an annual basis, as large-scale land-cover monitoring systems are rare [66] and continuous monitoring is often too cumbersome to conduct [67]. The accuracy of the NLCD is increasing with each installment [66,67,68], but this particular inadequacy is something to keep in mind when considering the results of this study.
Due to the aforementioned limitations and the fact that our results generally were not exceptionally strong statistical patterns, we propose that studies such as ours here serve as introductions to specific urban organismal and ecological systems. The data provided by eBird can be used to construct initial perceptions of how local environments impact a variety of species (i.e., more than a researcher or team could survey in the field on their own) and to track patterns widely over space and time (i.e., as the Phoenix area grows and may continue to experience water shortages). These frameworks may then be used to guide more refined site-specific (i.e., at the individual and group level) and trait-specific (both organismal and environmental traits) studies, beyond what citizen-science data can provide.
In summary, we revealed two environmental variables (proximity to cropland and proximity to a major river) that have recently shaped group-size variation in Cliff Swallows from a fast-growing western US metropolitan area. The fact that, over the past 5 years, Cliff Swallows formed larger groups closer to these types of habitat fits the prediction that access to food and water in the generally-resource-deprived desert is key to the survival and expansion of these birds into human-developed areas. Our result also works against the basic prediction that cities harm wildlife, and that certain native species may thrive based on unique, human-provided environmental assets (i.e., ‘urban oases’ [69]). We were able to uncover this using a vast community-science database, and as eBird usage continues to grow, we are excited about future abilities to ask these and related questions over time with more data.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/birds6020017/s1. Table S1: List of study years and associated swallow group-size observational data.

Author Contributions

C.R. co-designed the study, gathered and analyzed data, and wrote the manuscript. K.J.M. co-designed the study and edited the manuscript. All authors have read and agreed to the published version of the manuscript.

Funding

This material is based upon work supported by Barrett The Honors College at Arizona State University and the National Science Foundation under grant number(s) DEB-2224662, Central Arizona-Phoenix Long-Term Ecological Research Program (CAP LTER).

Institutional Review Board Statement

Not applicable.

Data Availability Statement

Data are publicly available in Dryad (https://doi.org/10.5061/dryad.c866t1gfd).

Acknowledgments

We acknowledge C.R.’s thesis committee members, Amy Frazier and Jamie Casseus, for offering significant help on this work. Additionally, we thank five anonymous referees and members of the McGraw Lab who provided input at various points throughout this research and manuscript preparation.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Locations of observation sites across the five years of the study (2018–2022), for Cliff Swallows reported in eBird from Maricopa County, Arizona, USA. For scale, Maricopa County stretches 212 km from west to east and 166 km from north to south.
Figure 1. Locations of observation sites across the five years of the study (2018–2022), for Cliff Swallows reported in eBird from Maricopa County, Arizona, USA. For scale, Maricopa County stretches 212 km from west to east and 166 km from north to south.
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Figure 2. Correlation matrix among predictors across 5 years of Cliff Swallow data. Legend shows number of years in which the correlation between two variables was significant (i.e., −4 equates to 4 years where the correlation was significant and negative). Values for only the upper-right half of the image are shown since bottom left below the diagonal line (in black) contains the same, mirror-image values. See text for variable abbreviations.
Figure 2. Correlation matrix among predictors across 5 years of Cliff Swallow data. Legend shows number of years in which the correlation between two variables was significant (i.e., −4 equates to 4 years where the correlation was significant and negative). Values for only the upper-right half of the image are shown since bottom left below the diagonal line (in black) contains the same, mirror-image values. See text for variable abbreviations.
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Figure 3. Relationships between group size in Cliff Swallows (abbreviated as “lnobs”, reflecting the natural-log-transformed observation data) and (a) eBird observation duration (abbreviated “lnduration”), (b) proximity (distance) to cropland (abbreviated “lnDisCro”), (c) proximity (distance) to a major river (abbreviated “lnDisMaR”), and (d) % impervious surface (abbreviated “FocalImp”). r2 = effect size.
Figure 3. Relationships between group size in Cliff Swallows (abbreviated as “lnobs”, reflecting the natural-log-transformed observation data) and (a) eBird observation duration (abbreviated “lnduration”), (b) proximity (distance) to cropland (abbreviated “lnDisCro”), (c) proximity (distance) to a major river (abbreviated “lnDisMaR”), and (d) % impervious surface (abbreviated “FocalImp”). r2 = effect size.
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Table 1. Data-source information for the tested urban and environmental parameters.
Table 1. Data-source information for the tested urban and environmental parameters.
SourceFile TypeSpatial ResolutionWhen Data Were CollectedAssociated Variables
PRISM Climate Data (Oregon State)Raster4 kmContinuousTemperature, precipitation
Census TIGER/Line GeodatabaseVector-2022Distance to primary/secondary road
Census Demographic DataVector-2020Population density, minority percentage
OpenStreetMap Water DataRaster3 s2021Distance to major river/canal/large lake
National Land Cover DatabaseRaster30 m2019Distance to open developed/low developed/medium developed/high developed/barren/forest/grassland/pasture/crops/woody wetlands/emergent herbaceous wetlands
Impervious Surface DataRaster30 m2019Percent impervious surface
Table 2. Global OLS model testing the effects of urban and environmental traits on group size in Cliff Swallows. See text for variable abbreviations; significant variables are shown in bold.
Table 2. Global OLS model testing the effects of urban and environmental traits on group size in Cliff Swallows. See text for variable abbreviations; significant variables are shown in bold.
VariableCoefficientt-Statisticp-ValueAdjusted R2AICc
lnduration0.226.250.000.102888.98
Temp−0.02−1.860.06
lnDisMaR−0.12−5.460.00
PPT−0.00−0.760.45
lnDisCro−0.10−5.990.00
percmin0.201.080.28
FocalForest−1.50−1.460.14
FocalImp−0.32−1.770.08
Table 3. Best-fit model testing the effects of urban and environmental characteristics on group size in Cliff Swallows. Percentage of impervious surface (FocalImp) does not make the best-fit model worse when removed (see text) and as such has been included.
Table 3. Best-fit model testing the effects of urban and environmental characteristics on group size in Cliff Swallows. Percentage of impervious surface (FocalImp) does not make the best-fit model worse when removed (see text) and as such has been included.
VariableCoefficientt-Statisticp-ValueAdjusted R2AICc
lnduration0.216.130.000.102885.40
lnDisMaR−0.12−5.650.00
lnDisCro−0.11−6.320.00
FocalImp−0.32−1.790.07
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Rueda, C.; McGraw, K.J. Urban Environmental Predictors of Group Size in Cliff Swallows (Petrochelidon pyrrhonota): A Test Using Community-Science Data. Birds 2025, 6, 17. https://doi.org/10.3390/birds6020017

AMA Style

Rueda C, McGraw KJ. Urban Environmental Predictors of Group Size in Cliff Swallows (Petrochelidon pyrrhonota): A Test Using Community-Science Data. Birds. 2025; 6(2):17. https://doi.org/10.3390/birds6020017

Chicago/Turabian Style

Rueda, Cassie, and Kevin J. McGraw. 2025. "Urban Environmental Predictors of Group Size in Cliff Swallows (Petrochelidon pyrrhonota): A Test Using Community-Science Data" Birds 6, no. 2: 17. https://doi.org/10.3390/birds6020017

APA Style

Rueda, C., & McGraw, K. J. (2025). Urban Environmental Predictors of Group Size in Cliff Swallows (Petrochelidon pyrrhonota): A Test Using Community-Science Data. Birds, 6(2), 17. https://doi.org/10.3390/birds6020017

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