Diet, phenology and body size shape nutrient release by songbirds

Abstract Animals can dramatically alter ecosystem structure and function through the cycling and transport of nutrients in their waste. While birds are particularly capable of influencing nutrient cycles due to their high mobility, abundance, metabolism and functional diversity, the factors shaping variation in nutrient release among birds remain poorly understood. We examined how trophic position, phenology and body size shape the release of carbon (C), nitrogen (N) and phosphorus (P) in the waste of a diverse assemblage of songbirds. We analysed waste samples (reflecting excretion and egestion) from 151 individual songbirds (31 species) at a migratory stopover site during one spring and one fall season in upstate New York, USA. Trophic position, represented by δ 15 N in waste, correlated positively with %N and negatively with C:P. Waste stoichiometry also differed between seasons with higher %N and lower C:N in spring compared to fall as birds shifted their diets from N‐rich insects in spring to C‐rich fruits and seeds in fall. Birds in the fall exhibited increased fattiness, and body size had the strongest influence on C, N and C:N release during this season, suggesting that fat accumulation for energy storage may shape nutrient excretion prior to migration. However, the effects of body mass and its interactions with season and trophic position were complex. Ultimately, trophic position, phenology and body mass together helped explain variation in nutrient release among songbirds. Future research may leverage this information to further explore how migratory songbirds influence stopover habitats and other ecosystems via contributions to nutrient cycling. Read the free Plain Language Summary for this article on the Journal blog.