The dual lens of diversity: Seasonal variability of avian community structure in urban hills

Urbanization has significantly increased the environmental intricacy of low mountainous and hilly urban landscapes, exacerbating the need for biodiversity conservation in these ecosystems. This study involved comprehensive bird surveys across four seasons from July 2022 to April 2023 along 64 transects in the low mountainous terrain of Jiangsu Province. Adopting a dual lens of taxonomic and functional dimensions, our research explored seasonal fluctuations and assembly mechanisms governing avian community diversity (both α and β) on these urban hills. The results showed that species richness and functional diversity (FD) were higher in spring and lower in autumn, whereas functional mean pairwise distance (MFD) and mean nearest functional distance (FD.MNTD) were higher in summer and winter. Although the standardized effect sizes for MFD were not significantly different from zero in any of the four periods, those for FD.MNTD were significantly less than zero in summer, autumn, and winter, indicating a propensity for functional trait clustering within avian communities during these seasons. Taxonomic variations were found to be chiefly governed by species turnover, whereas functional dynamics were principally shaped by nestedness. A marked disparity emerged, with functional β-diversity and its turnover components consistently registering lower values than their taxonomic counterparts, accompanied by a higher level of functional nestedness versus taxonomic nestedness. This study underscores the intricate seasonal dynamics of avian communities in urbanized low mountainous regions and provides crucial insights to devise conservation strategies aimed at preserving avian biodiversity within these ecologically complex urban landscapes.