Adapting to urban heights: Flexible nest-site selection strategies of a human commensal bird along climate and anthropogenic gradients

Urbanization is reshaping ecosystems worldwide, driving wildlife to
navigate and adapt to novel and highly dynamic environments. The Eurasian
tree sparrow (Passer montanus, ETS) serves as an exemplary human
commensal, thriving in cities through exceptional behavioral and
ecological flexibility. Here, we systematically investigated the nest-site
selection strategies of ETSs across 645 buildings across 22 cities in
northern China, integrating climatic, geographic, biotic, and
anthropogenic variables at a macroecological scale. We found that both the
availability and use of nest sites increased with building height,
underscoring ETSs’ capacity to exploit vertical resources in dense urban
landscapes. Notably, the preference for lower nest heights when nest sites
were abundant suggests a strategy to reduce intraspecific competition and
energy expenditure. Negative associations between nest-site use or
preference and the normalized difference vegetation index indicate that
ETSs favor anthropogenic over vegetated resources, likely to circumvent
interspecific competition in urban green spaces. Additionally, altitudinal
gradients modulated ETSs’ nesting responses: at lower elevations, higher
building heights promoted nesting, whereas increased economic development
(gross domestic product per cell) and noise suppressed it—signaling an
avoidance of intense anthropogenic disturbance. Conversely, ETSs showed
reduced competition at higher altitudes and increasingly relied on
resources linked to urban prosperity. These findings reveal a nuanced
behavioral adaptability in ETSs, allowing them to navigate trade-offs
between anthropogenic benefits and environmental stressors across spatial
gradients. Our study offers critical insights into the eco-behavioral
mechanisms underlying urban adaptation and the evolution of commensalism,
with important implications for biodiversity management and sustainable
urban design.