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Aim: Our objective was to quantify urban tolerance for North American birds across the full annual cycle. We tested (1) whether intra-annual variability of urban tolerance differed between migrants and residents and (2) whether intra-annual variability of urban tolerance was phylogenetically conserved. We then assessed how the relationship between ecological and life history traits and urban tolerance differed both across the year and between migrants and residents. Location: North America. Taxon: Birds. Methods: We integrated a large citizen science dataset of observations for 237 bird species, remotely-sensed VIIRS night-time lights data, and trait data on each species. We estimate, for each species and each month of the year, a continuous measure of urban tolerance (i.e. the median of their distribution of observations across an urbanization gradient). We then use phylogenetic linear models to assess the relationship between this measure of urban tolerance and various life history and ecological traits. Results: There was a distinct drop in the overall urban tolerance scores corresponding with the breeding period; this pattern was more pronounced for migrants compared to residents. Migrants also had greater intra-annual variability than resident species. We also found that the strength of the relationships between ecological and life history traits and urban tolerance was highly seasonal for most traits considered, and some divergent patterns were noted between migrants and residents. Main conclusions: The urban tolerance of birds greatly changed throughout the annual cycle, with different patterns for migrants and residents. Compared to residents, migrants showed more intra-annual variability of urban tolerance with a drop in the average urban tolerance score during the breeding season. Together, our results suggest that urban tolerance is a function of both species and season, and they highlight the importance of considering the dynamic nature of birds’ use of urban ecosystems throughout the full annual cycle.

Cities are novel environments compared with the evolutionary history of the species that reside within them. Collectively, cities and their fauna can be thought of as ecosystems, recognized as playing a critical role in supporting global biodiversity, but they are fundamentally a combination of old species surviving or thriving in a new environment, and the mechanisms and underlying processes which support biodiversity within cities have not been investigated at broad macroecological scales. We aimed to understand — at a broad macroecological scale — how biodiversity responds both among and within cities. We integrated > 5 million eBird citizen science observations with remotely sensed landcover products throughout 1,581 cities within the continental United States. We first investigated the species-area relationship as it pertains to cities and compared the slope of this relationship to randomly sampled polygons. Second, we investigated how biodiversity responds to an urbanization gradient at the level of localized bird observations. We found strong support for the longstanding species-area relationship theory: geographically larger cities had greater species richness. Surprisingly, the species-area relationship was greater in cities when compared to the overall relationship for randomly sampled polygons in the study region (continental United States), which included many different land use and land cover types. Our finding suggests that diverse and heterogeneous cities play a significant role in supporting biodiversity. We also found that there is a consistent threshold where the level of urbanization begins to profoundly and negatively affect biodiversity. Critically, urban planning at the city-scale and at a local-scale (e.g., neighborhood) should focus on preserving attributes of water and tree-cover for increased biodiversity to keep as much of the city as possible above this threshold value.

Understanding species-specific relationships with their environment is essential for ecology, biogeography, and conservation biology. Moreover, understanding how these relationships change with spatial scale is critical to mitigating potential threats to biodiversity. But methods which measure inter-specific variation in responses to environmental parameters, generalizable across multiple spatial scales, are lacking. We used broad-scale citizen science data, over a continental scale, integrated with remotely-sensed products, to produce a measure of response to urbanization for a given species at a continental-scale. We then compared these responses to modelled responses to urbanization at a local-scale, based on systematic sampling within a series of small cities. For 49 species which had sufficient data for modelling, we found a significant relationship (R2 = 0.51) between continental-scale urbanness and local-scale urbanness. Our results suggest that continental-scale responses are representative of small-scale responses to urbanization. We also found that relatively few citizen science observations (~250) are necessary for reliable estimates of continental-scale urban scores to predict local-scale response to urbanization. Our method of producing species-specific urban scores is robust and can be generalized to other taxa and other environmental variables with relative ease.