Breeding in the City: Non-Linear Associations of Urbanization, Tree Cover, and Human Disturbance with Eurasian Tree Sparrows (Passer montanus) Breeding Performance

Urbanization can alter habitat structure and disturbance regimes, with consequences for bird reproduction. We used 13 years of nest-box monitoring data from 15 urban green spaces in Madrid, Spain, with an average of 866 nest boxes monitored annually, to assess how urbanization, tree cover, building height, road infrastructure, noise, and pedestrian intensity were associated with Eurasian Tree Sparrow (Passer montanus) breeding performance. Nest-box occupancy, eggs and hatched chicks per box, nestlings reaching ringing age, and mean number of clutches showed non-linear associations with urbanization, with the highest values generally recorded in partially urbanized areas. Occupancy was 66.9% in these areas, compared with 17.5% in urbanized areas. Tree cover was positively associated with productivity in highly urbanized areas, suggesting that vegetation may be particularly relevant to productivity under highly urbanized conditions. In contrast, greater building height was associated with lower occupancy and reproductive performance. Pedestrian intensity was negatively related to nest-box use and reproductive output, whereas road infrastructure and noise mainly affected later reproductive stages. Overall, breeding performance was associated with the combined characteristics of vegetation, urban structure, and human disturbance. Maintaining partially urbanized green spaces and increasing tree cover in densely built areas may be associated with more favourable breeding conditions for Eurasian Tree Sparrows.