Corey Thomas Callaghan, Yanina Benedetti, John Wilshire, Federico Morelli

Generalist species — with their wide niche breadths — are often associated with urban environments, while specialist species are likely to be most at-risk of increasing urbanization processes. But studies which quantify the relationship between trait specialization (i.e., niche breadth) and urban tolerance are generally methodologically limited, with repeatable robust methods to easily quantify this relationship among different regions and time scales often lacking. Our objective was to use novel methods to quantify the relationship between trait specialization and urban tolerance over a broad spatial scale. We used ~ 2 million citizen science observations and spatially intersected these with remotely-sensed VIIRS night-time light values and a novel continuous measure of a species’ trait specialization for 256 European bird species. We found a negative relationship between avian urban tolerance and an overall specialization index. Nesting site niche breadth was especially negatively associated with higher urban tolerance scores. Our results highlight that species with a high degree of trait specialization likely have a lower capacity to persist in urban ecosystems, and hence, could be most at-risk in novel urban ecosystems. We suggest that trait specialization can be used as a proxy for the degree of risk posed by urban environments to a given species.