Seasonality of Functional Distinctiveness in Global Avifauna

Functionally distinct species play outsized roles in ecological assemblages, as their unique characteristics often underpin key ecosystem functions. Although global patterns of functional distinctiveness have been documented, how these patterns vary seasonally remains unexplored, despite strong seasonal dynamics in many taxa. We present the first global assessment of seasonal variation in avian functional distinctiveness, examining local and global distinctiveness at both species and assemblage levels and contrasting migratory and resident species. At the assemblage-level, functional distinctiveness increases with distance from the equator, with the largest seasonal changes observed in boreal and Arctic regions. These patterns arise because migratory and resident birds contribute differently to functional diversity: migratory species are more functionally redundant than residents, leading to reduced assemblage distinctiveness upon migrants’ arrival. At the species-level, functional distinctiveness within local assemblages varies little between seasons and broadly correlates with species’ distinctiveness within the global avifauna. Our analysis also suggests that traits associated with pelagic specialization correlate most strongly with species’ global distinctiveness, while ground-foraging and nocturnality tend to underlie distinctiveness within local assemblages. Our results highlight the importance of migration in shaping spatial patterns of functional distinctiveness and reveal how migrants and residents contribute to assemblage functional diversity throughout the year.