Urbanisation and Habitat Shape Resource-Driven Dietary Shifts in Wild Birds

Urbanisation affects bird ecology and evolution, with changes in nutritional intake considered a key driver. However, most studies provide only snapshots of urban bird feeding ecology due to methodological limitations in analysing diets across space and time. Here, we address this gap by examining the diets of two common species with differing feeding ecologies: the great tit (Parus major) and blue tit (Cyanistes caeruleus). Diet samples from 370 birds, captured over a year at 147 locations across an urbanised landscape in Innsbruck, Austria, were molecularly analysed. Results revealed species- and season-specific diet patterns influenced by urbanisation: urban great tits exhibited higher diet diversity but reduced arthropod consumption during the breeding season, while urban blue tits compensated for lower moth intake by increasing their consumption of crab spiders and aphids. Prey consumption mirrored prey availability, highlighting resource-driven dietary shifts. Habitat type also played a significant role, with urban green spaces enhancing plant-based food diversity and residential areas increasing anthropogenic food consumption. These findings support the hypothesis that diet drives fitness and phenotypic differences between urban and rural bird populations. They highlight the need to consider both urbanisation levels and habitat characteristics to fully understand its ecological and evolutionary impacts.