Adapting to Urban Heights: Multidimensional Gradients Drive Nest-Selection Plasticity and Trade-Offs in a...

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 residential 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 height preference 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 context-dependent patterns of nest-site selection in ETSs across multidimensional urban gradients. Our study documents behavioral plasticity in a highly successful urban-commensal species and highlights the need for future research linking such patterns to individual fitness, population persistence, and broader biodiversity management in cities.