City life anticipates the breeding of a bird of prey without affecting its reproductive success

Urbanisation poses a profound threat to biodiversity, leading to the loss of natural ecosystems and changes in animal communities. Many species of birds of prey are increasingly associated with urban habitats even when they have low reproductive success. However, it is unclear if this poor reproductive performance is due to the worse environmental conditions of the cities or to the poorer quality of the nesting sites. Addressing the effects of urban habitat on reproduction under similar nesting conditions (nest-boxes of same size and material) is therefore important if we were to test a direct effect of the habitat quality on cavity-nesting raptors. To address this question, we compared life-history traits, metrics of reproductive success, and morphological traits of chicks of common kestrels (Falco tinnunculus) breeding in artificial nest boxes in the city of Rome, and in nearby rural and natural environments over a period of five years. We found that kestrels laid significantly earlier in the city (10 days on average) than in the natural habitat. We also found novel evidence that chicks in rural habitats had shorter wings compared to chicks raised in urban habitats (1.2 cm on average). By contrast, we did not detect any differences in clutch size, brood size at fledging, egg volume, hatching success, fledging success, and body mass, tarsus length, and body condition of chicks among breeding habitats. Our findings suggest that, despite the differences in breeding phenology, kestrels had similar reproductive performances across different habitat types. This result is in contrast with previous studies on the same species carried out in other European cities, indicating that some urban habitats might be optimal for sustaining viable bird populations.