Nest site and nesting substrate selection by House Sparrows along an urban gradient in Nuwara Eliya, Sri Lanka

The House Sparrow (Passer domesticus) is an obligate synanthrope experiencing significant population declines across its global range, yet remains locally abundant in certain urban centers. Understanding the drivers of these robust populations is critical for urban avian conservation. We assessed the abundance and nest site selection of House Sparrows across an urban-to-exurban gradient in Nuwara Eliya, Sri Lanka. Data was collected from 20 plots, each representing one of four habitat types. We employed a Negative Binomial Generalized Linear Model to account for overdispersion of data and used Pearson’s Chi-square tests to analyze the associations between nest substrate selection, habitat, and building architecture. A total of 185 active nests were recorded. Nest abundance did not vary significantly across habitat types (X² = 3.49, p = 0.32) or along building cover (Z = 0.72, p = 0.47) and green cover (Z = -1.78, p = 0.08) gradients, indicating a ubiquitous distribution. However, nest substrate selection was highly non-random and was more strongly associated with building architecture (X² = 178.3, p < 0.001) than with broad habitat categories (X²= 47.34, p < 0.001). Nest boxes were the primary substrate on residential houses (n=45), while commercial buildings and shops provided diverse opportunistic sites such as roofing sheets and wall crevices. The House Sparrow population in Nuwara Eliya is stable and resilient, likely sustained by a combination of traditional human provisioning (nest boxes) and the species’ plasticity in exploiting modern architectural features. Our findings suggest that for this species, immediate structural characteristics of the built environment are more significant drivers of occupancy than macro-habitat urbanization labels. This study provides a vital baseline for avian conservation in Sri Lanka and highlights the role of anthropogenic micro-habitats in maintaining urban biodiversity.