Urban vultures preferentially roost at sites surrounded by more developed land cover and lower deer carcass density.

Land cover changes resulting from urbanization alter habitat structure and resource availability. Vultures provide ecosystem services such as nutrient cycling and may help limit disease transmission, making them an important feature of urban areas. Urbanization may have positive and negative impacts on vultures, such as increasing foraging opportunities and decreasing nesting success, complicating our understanding of the effect of urbanization on these species. We examined how local and landscape features affect roost attendance of black vultures (Coragyps atratus) and turkey vultures (Cathartes aura) to better understand the factors that play a role in site selection and habitat and landscape use. We counted the number of vultures at twenty-nine roosting sites in the Charlotte Metropolitan Area, NC once a month between November and March over two years. At each roosting site, we measured roosting structure height, vegetation height, and open space, and calculated the amounts of developed land cover, open water, developed-forest edge density, and deer carcass density in surrounding landscapes of 0.4 to 20km radii. The best model for roost attendance included wind speed, vegetation height, developed land cover within 15km, developed-forest edge density within 15km, deer carcass density within 15 and 20km, and survey date. Developed land cover was associated with higher roost attendance while other variables were associated with lower roost attendance. The effects of landscape variables on roost attendance suggests urban vultures are relying more on trash and anthropogenic food sources, which may alter nutrient cycling, disease dynamics, and reproductive success. each we estimated using a scale from in 10% increments. Our results indicate that urban vulture roost attendance depends on local weather and habitat conditions, and landscape structure and deer carcass density at large spatial scales. More vultures were counted at roosting sites adjacent to lower vegetation heights and surrounded by landscapes with more developed land cover, less developed-forest edge density, and lower deer carcass density. More vultures were also counted at roosting sites when wind speed was lower. In the following, we discuss the possible reasons underlying the effects of local habitat and weather conditions on vulture roost attendance and posit that the effects of landscape variables on roost attendance suggest that urban vulture populations may be relying more on anthropogenic food sources than on roadkill and possibly other natural food sources. We conclude by discussing the implications of our results for the management of urban vulture populations.


Introduction
By 2040, the global human population is projected to be greater than 9 billion, with 68% of people living in urban areas (United Nations 2019). Populations growing at these rates require more infrastructure, bringing increased levels of tra c, pollution, and disturbance to natural environments and wildlife populations. Land cover changes that result from this increasing urbanization reduce and degrade critical bird habitats on local and global scales (Isaksson 2018). Many species cannot persist in these altered conditions, leading to decreased diversity and few successful species in highly urbanized areas (Faeth et al. 2011).
Vultures within the Cathartidae family include several of these successful species, to the point that they are often seen as nuisances in cities (Blackwell et al. 2007). Cathartid vultures have adapted to cities well, using human-made structures for roosting, foraging, and nesting across North and South America (Avery et al. 2002;Coles 1944;Hill III and Neto1991). Historically, vultures roosted on trees and other natural structures in largely undeveloped areas (Coleman and Fraser 1989;McVey et al. 2008; Rabenold and Decker 1990). Today, urban vulture roosts are commonly found on arti cial structures such as transmission towers, cellular towers, and water towers across the United States, representing a dramatic change in behavior over a relatively short time span (Avery et al. 2002;Seamans 2004). Vultures also bene t from roadkill, land lls, and residential land cover in cities for foraging (Novaes and Cintra 2015; Thompson et al. 1990), although foraging opportunities may be dependent on sanitation practices, which can change over time (Coleman and Fraser 1989;Houston et al. 2007;Stewart 1974). Vultures would naturally nest in caves and tree cavities but have begun nesting in abandoned buildings and other human structures, making them more vulnerable to habitat loss and disturbance during the nesting season, which can dramatically reduce nest success (Coleman and  In our study area in the southeastern United States, black vultures (Coragyps atratus) and turkey vultures (Cathartes aura) are common sights in urbanized areas, despite previous population declines (Robbins et al. 1989). Previous research has reported black vultures occurring near foraging sites, such as street markets and garbage dumps, possibly to reduce movement costs in developed landscapes (Novaes and Cintra 2013). Campbell (2014) found that black vulture numbers show strong positive associations with highly urbanized areas, being commonly found in city centers and suburbs, likely due to increased food availability in these areas (Campbell 2014). Turkey vultures are also associated with urbanized areas, but to a lesser extent than black vultures as they tend to range farther and forage on less predictable sources (Campbell 2014).
Vultures are specialized for scavenging, with e cient soaring ight, bald heads, and extremely corrosive stomach acid that allows them to consume carcasses infected with diseases without becoming infected themselves (Ogada, Keesing, et al. 2012). As such, vultures carry out a vital function in urban ecosystems by disposing of carcasses and organic matter that could otherwise spread deadly diseases to other scavengers and ultimately human populations. By scavenging, vultures also play an important role in nutrient cycling at the relatively large spatial scales over which they forage (Hill et al. 2018).
Despite the importance of vultures to ecosystem functioning and their increasing prevalence in urban landscapes, we know very little about the factors underlying their success. Our objectives for this study were to assess the impact of several local and landscape features on vulture roost attendance. In the Charlotte Metropolitan Area, we used vulture roost surveys, local habitat surveys, and geospatial analyses to evaluate the effects of time and weather conditions, local habitat structure, landscape structure, and landscape deer carcass density on the roost attendance of black and turkey vultures at 29 roosting sites over two years. Based on previous studies and personal observations, we expect developed land cover to positively impact roost attendance due to vultures readily taking advantage of anthropogenic structures and resources.

Roosting sites
We identi ed 15 black vulture and turkey vulture communal roosting sites within the CMA in 2019 and an additional 14 in 2020 for a total of 29 roosting sites (Fig. 1). Selected roosting sites were located by the lead author using a list compiled by Mecklenburg Audubon Society (MAS) members, eBird reports, and personal observations of vulture movements across the CMA at sunrise and sunset (totaling approximately 80 hours).
Our use of multiple sources of information to locate roosting sites helped to maximize roosting site sample size and minimize any potential spatial bias in roost location due to the preferential selection of easily visible sites near roads and/or other development. Prior to being included in the study, all reported and observed roosting sites were checked for vulture presence and type of roost. We selected roosting sites that hosted vultures overnight and excluded temporary roosts used during the day as resting and congregating sites prior to or immediately following overnight roosting.
Twenty-seven roosts were located on transmission or cellular towers. Roosts on transmission towers (22) were situated in right-of-way (ROW) corridors with forest on either side, whereas roosts on cellular towers (5) were not located in right-of-way corridors and were adjacent to a variety of land covers. The two roosts not located on towers were on small clusters of deciduous trees within 1km of transmission towers and adjacent to residential development.

Roost surveys
We counted the number of vultures at each roost once a month from November 2019 to March 2020 and from November 2020 to March 2021, for a total of ve surveys per roost per year. Thirteen roosts were surveyed in both years and 16 roosts were surveyed in a single year. Survey periods coincided with the non-nesting season during which vultures use roosts most consistently by returning to the same roost night after night and in the largest numbers (Sweeney 1984). During each survey, we counted vultures at roosts between 30 minutes before and 30 minutes after sunrise when individual vultures could be distinguished as they became more active and spread out on the roosting structure but before they left the structure to forage in the surrounding landscape (Sweeney 1984). During the second year of surveys, the count period was shortened to only the 30 minutes before sunrise as a result of observations in the previous year that vultures often left roosting sites earlier than expected (see the Analyses section below for the correction applied to rst year data to account for this difference in survey methods). Trained volunteers assisted the authors in conducting surveys in both years.

Explanatory variables
Local variables We measured local weather and habitat variables at each roosting site that may be important in explaining vulture attendance including air temperature, wind speed, cloud cover, roosting structure height, surrounding vegetation height, and open space (Table 1). Vultures generally remain at or near roosting sites longer during colder temperatures or inclement weather (Sweeney and Fraser 1986). However, on windy mornings, vultures may be more likely to leave the roost earlier, perhaps to take advantage of wind currents for early-morning foraging ights (Davis 1979). We measured roosting structure height, surrounding vegetation height, and open space to account for the physical characteristics of roosting sites that may aid in arrival, departure, and ight ease. Open elds or corridors surrounding roosting sites allow unobstructed arrival and departure and may provide upward air currents (Coleman and Fraser 1989;Davis 1979). The same bene ts may be provided by the height of the roosting structure, with structures above tree level providing easier access.  We corrected for any bias resulting from vultures leaving roosting sites earlier than expected by conducting additional counts of roosting vultures in relation to time before and after sunrise to estimate the rate at which vultures leave roosting sites. The lead author counted the number of vultures roosting every minute from 60 minutes prior to sunrise to 60 minutes after sunrise at eight different roosting sites over 16 days from September 2019-March 2020. We then used a 6th order polynomial regression to model the proportion of the roost remaining with respect to time before and after sunrise (F 6,1042 = 262.30, p < 0.001, adjusted R 2 = 0.60; Figure A1). All roost counts from 2019-2020 and 2020-2021 were adjusted by dividing the count by the predicted proportion of the roost remaining at the time the count was conducted.
We identi ed the local and landscape features associated with the adjusted number of vultures at roosts using repeated measures, generalized linear mixed-effects models, multi-model inference, and simultaneous autoregressive models. All models included a random site effect to account for the non-independence of observations from the same roosting site. Models also included survey date, measured as Julian date, i.e., the number of days since the beginning of the Julian period in 4713 BC, to account for variation in roost attendance across months and years (Sweeney and Fraser 1986). We restricted landscape variables in models to those from the same spatial scale in order to minimize high levels of collinearity among explanatory variables. Collinearity among local and landscape variables in models was below thresholds above which levels are deemed unacceptable (r < 0.70 and VIF < 10 (Dormann et al. 2013); Appendix A), except for the correlations between Developed-Forest edge density and Developed land cover within 10km, 15km, and 20km (0.72 ≤ r ≤ 0.79) (Tables A1-A11). We divided explanatory variables by their partial standard deviations (standard deviations divided by VIF values, sample size, and the number of predictor variables (Cade 2015)) and we also standardized explanatory variables. Finally, we log-transformed the response variable in models to address heteroskedasticity in residuals.
We evaluated models containing all possible combinations of explanatory variables, with the exception that models contained only landscape variables from the same spatial scale, using AIC c . As roosting sites were spatially clumped in our study area, we tested for spatial autocorrelation in the residuals of the best models (ΔAIC c < 2) (Burnham and Anderson 2002) using correlograms, Moran's I, and Bonferroni-corrected p-values.
Most residuals exhibited signi cant spatial autocorrelation (Figures A2-A8). To account for this source of variation, we re-analyzed the top models as simultaneous autoregressive models of the spatial error type (SAR err ) using the distance at which spatial autocorrelation was most pronounced in correlograms to de ne neighborhoods. We

Results
Seven models quali ed as the best models (ΔAIC c < 2) describing vulture roost attendance ( Table 2). Models included wind speed, surrounding vegetation height, Developed land cover within 15km of roosting sites, Developed-Forest edge density within 15km of roosting sites, deer carcass density within 15km and 20km of roosting sites, and survey date. The number of vultures was larger at roosting sites bordered by lower vegetation and surrounded by landscapes with lower carcass densities, lower developed-forest edge densities, and more developed land cover (Figs. 2-3). Table 2 The top models (ΔAIC c < 2) describing black vulture (Coragyps atratus) and turkey vulture (Cathartes aura) roost attendance in the Charlotte Metropolitan Area, USA.
Variables include wind speed (WIND), surrounding vegetation height (VEG), the amount of developed land cover within 15km of roosting sites (DEV_15), developedforest edge density within 15km of roosting sites (EDGE_15), deer carcass density within 15km of roosting sites (DEER_15) or 20km of roosting sites (DEER_20), and survey date (DATE).

Discussion
Our results indicate that urban vulture roost attendance depends on local weather and habitat conditions, and landscape structure and deer carcass density at large spatial scales. More vultures were counted at roosting sites adjacent to lower vegetation heights and surrounded by landscapes with more developed land cover, less developed-forest edge density, and lower deer carcass density. More vultures were also counted at roosting sites when wind speed was lower. In the following, we discuss the possible reasons underlying the effects of local habitat and weather conditions on vulture roost attendance and posit that the effects of landscape variables on roost attendance suggest that urban vulture populations may be relying more on anthropogenic food sources than on roadkill and possibly other natural food sources. We conclude by discussing the implications of our results for the management of urban vulture populations.
Surrounding vegetation height and wind speed were each negatively associated with vulture roost attendance in our study area. Shorter vegetation surrounding the roosting site may allow vultures easier access to the roosting structure and make departure ights easier. Vultures at many of our sites ew directly from the roost to a nearby foraging area before sunrise. Easy entry and departure from the roosting site, made possible by shorter vegetation, may facilitate such directed movements and thus play a role in vulture roosting site selection. The negative effect of wind speed on vulture roost attendance in our study is supported by prior evidence suggesting that vultures choose favorable microclimates at roosting sites that reduce wind speed (Thompson et al. 1990). Future studies should employ anemometers installed on roosting structures to test this hypothesis. Vultures in our study area may be relying more on trash and other resources in developed land cover because of the relative abundance of these resources and because of the risk of mortality associated with foraging on Assuming that food availability is the ultimate driver of roost attendance in the non-breeding season, the identi cation of suitable roosting sites in landscapes with these characteristics may enable managers to implement prevention measures, such as limiting the availability of trash before a con ict arises.

Conclusion
As urban areas continue to expand, it is important to understand how wildlife populations respond to changes in habitat structure and resource availability. We found that vulture roost attendance is negatively affected by wind speed and surrounding vegetation height at roosting sites and developed-forest edge density and deer carcass density in landscapes, and positively affected by the amount of developed land cover in landscapes. Importantly, these results imply that urban vultures are relying more on anthropogenic sources of food available in developed areas and less on roadkill and possibly other natural food sources. Future research should analyze the diet composition of urban and rural vultures to attempt to identify their primary food sources and to better understand the broader impacts that dietary changes may have on carcass decomposition, disease transmission, and pollution patterns in cities. Future research should also examine the species-speci c responses of black and turkey vultures to habitat structure and resource availability in urban regions and the management strategies needed to prevent human-vulture con icts. A deeper understanding of vulture distribution and occurrence in cities and their drivers will signi cantly advance the coexistence of these important species with humans. Black vulture (Coragyps atratus) and turkey vulture (Cathartes aura) communal roosting sites (29) within the Charlotte Metropolitan Area, USA, labelled by roosting structure type. Three areas with overlapping roost locations are shown as insets; other roost locations may overlap in the main map. The green shading represents vegetated areas, the gray represents developed areas, and the major water bodies are shown in blue Figure 2 The effects of survey date and local explanatory variables in the top models (ΔAIC c < 2) describing black vulture (Coragyps atratus) and turkey vulture (Cathartes aura) roost attendance in the Charlotte Metropolitan Area, USA. Model-averaged standardized coe cients ± 2 SE are in parentheses below variable names