Corey Thomas Callaghan, William K Cornwell, Alistair G. B. Poore, Yanina Benedetti, Federico Morelli

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.