Changes in bird occupancy and its association with functional traits under long-term context of urbanization and suburbanization in a metropolis

Urbanization is one of the key factors contributing to biodiversity decline. The urban–rural gradient is commonly used to understand the impact of urbanization on bird communities. However, there is limited long-term research on community changes in the context of urbanization and suburbanization. Furthermore, investigating the relationship between species occupancy change and functional traits can aid in predicting bird responses to urban environments and developing targeted conservation strategies. Our study focused on Shanghai, a highly urbanized city in China, and had the following objectives: (1) estimating bird occupancy changes in urban and suburban areas; (2) analyzing the environmental factors contributing to the estimation of occupancy probabilities; (3) investigating the relationship between occupancy changes and functional traits. We employed a multi-species occupancy model to calculate bird occupancy probabilities and used multiple regression and generalized additive models to examine objective 3. The results indicated a slight increase in overall bird occupancy in the central urban area and a slight decrease in the suburban area. Climate and land use were important factors in estimating occupancy probabilities. Vegetation cover had a positive effect on occupancy, while impervious surfaces had a negative impact. Increasing vegetation cover in central urban areas and the expansion of impervious surfaces in suburban areas might contribute to fluctuations in occupancy probabilities. Small bird species exhibited significant variability in occupancy changes, while medium-sized species generally showed an increase, and large species exhibited a decrease in occupancy probability. Furthermore, the increase in occupancy probabilities of resident birds in urban centers was more pronounced compared to migratory birds, whereas no such statistically significant differences were observed in the suburbs. Our findings provided valuable insights for bird conservation and emphasized the necessity of integrated urban-suburban biodiversity conservation and the consideration of long-term impacts of urban expansion in planning processes.