Spatial and seasonal variation in avian dietary strategies

André M. Bellvé, Reymond J. Miyajima, Marta A. Jarzyna

Diet is a fundamental aspect of vertebrate life history, shaping survival, recruitment, and ultimately fitness. While spatial variation in avian dietary traits has been extensively studied, seasonal dynamics at both species and assemblage levels remain largely unexplored, hindering our ability to uncover the ecological and evolutionary mechanisms underlying biodiversity patterns. Here, we present the first global-scale assessment of seasonal variation in avian dietary space and its environmental drivers, integrating seasonal species distributions for over 10,000 bird species with the database of intra-annual variability in avian dietary preferences. We show strong seasonal variation in birds’ dietary space at both the assemblage and species levels on a global scale, with most pronounced intra-annual diet variability in the temperate and boreal regions of the Northern Hemisphere. We show that this seasonality arises from two key processes: (1) the seasonal redistribution of migratory species, which occupy distinct regions of dietary space, alters assemblage composition and thus dietary space, and (2) within-species dietary shifts, particularly pronounced among migratory birds. Viewing diet and other species’ traits as dynamic systems provides a powerful framework to better capture the temporal complexity of trait-environment associations, understand factors shaping community structure, and advance conservation efforts.