{Avian trait specialization is negatively associated with urban tolerance}

NOTE: This is a pre-print, still under consideration for publication, and this version has not undergone peer review. ABSTRACT 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 spatially, temporally, and methodologically limited. We integrated $\sim$ 12 million citizen science observations with remotely-sensed VIIRS night-time light values and a novel continuous measure of a species' trait specialization for 278 European bird species. We found a negative relationship between avian urban tolerance and an overall specialization index. Specifically, specialization of foraging substrate and nesting site niche breadth were both negatively associated with higher urban tolerance scores. Our results highlight that species with a high degree of trait specialization (i.e., small niche breadth) have a decreased capacity of adaptation to 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.