City birds are smaller but noisier: morphology, body condition, and song variation between Rufous-collared Sparrows (Zonotrichia capensis) from urban and wild environments in Central Chile

Urban environments impose challenges for wildlife survival, yet some populations show morphological and behavioral adaptations to these novel conditions. The Rufous-collared Sparrow, Zonotrichia capensis, is a neotropical bird that successfully inhabits both urban and wild environments. This study evaluated morphological and song variations of populations in Central Chile and their association with food availability and anthropogenic noise. We measured 34 individuals from the urban and 64 from the wild environment and recorded 46 songs during the breeding season. Food availability and fragmentation were evaluated through primary productivity (NDVI), alongside environmental noise levels. Results show that individuals inhabiting the urban environment have a smaller size, lower body mass, and lower body condition than their conspecifics from wild environments. Furthermore, song structure shows an increase in the minimum frequencies in the urban population, where we registered higher noise levels. These findings suggest that continuous food availability, higher fragmentation, and anthropogenic noise in the city explain the morphological and vocal divergence observed in urban populations compared to wild ones.