COS 92-2 - Who can afford an urban life? Birds’ adaptation to cities is determined by what they eat and where they breed

Abstract: The urban environment is highly shaped by human activities and imposes an extreme challenge on wildlife living inside. Whether a species can live in cities depends on the traits it possesses. However, only few studies have examined impacts of urbanization on the composition of species’ traits within avian assemblages in cities across large environmental and socioeconomic gradients. Hence, the aim of this study was to understand which traits affect birds’ adaptation to the urban environment and which trait types can help birds live in cities. We examined the compositions of the traits closely associated with birds’ survival and breeding success within the assemblages in 106 cities worldwide. By integrating information from over 30 million species occurrence records, species range maps, and foraging and nesting traits of over 4,000 bird species, we compared the trait compositions of the bird assemblage actually observed in a city to those of the hypothetical assemblage that would be expected in the same area in the absence of urbanization. From all bird species whose breeding ranges overlapping a city, we randomly selected the same number of species as the observed assemblage in that city to obtain the expected assemblage. We repeated this process 1,000 times and calculated the mean difference in the prevalence of each trait type between the observed and expected assemblages as an indicator of the adaptation of that trait type to the urban environment. We found that scavengers, frugivores and nectivores were over-represented in the observed assemblages compared to the expected ones. This indicates that having the ability to use additional food sources provided by people (e.g., food waste and gardening plants) may be important for birds living in cities. Birds that build their nests on trees and cliffs had higher representation, while those building on the ground had lower representation than expected. Additionally, birds building enclosed nests (i.e., domed nests, secondary cavities) were found to be more adapted to the urban environment than those building open nests (i.e., scrape nests). Since enclosed nests on trees or cliffs may face lower intensity of human disturbances than open nests on the ground do, the results suggest that the nest characteristics that can reduce the influence of human activities on eggs and chicks essentially affect birds’ adaptation to the urban environment. The new insights into how birds adapt to the urban environment provide essential information to achieve a sustainable city.