Animals can dramatically alter ecosystem structure and function through nutrient cycling via waste. This is particularly true for birds, which display incredible capacity as dispersal agents due to their high mobility, metabolism, and ecological diversity. However, our ability to predict avian impacts on ecosystem nutrient dynamics is currently inhibited by a limited understanding of how birds vary in the nutrients they release and what drives these differences. In this study, we examined variation in the full elemental composition of bird waste across 38 species and two seasons (fall and spring). We hypothesized that dietary differences drive variation in nutrient release among birds and are influenced by age and season, which corresponds with migration timing. We collected fecal data from birds banded at the Braddock Bay Bird Observatory in NY, USA during fall and spring of 2021–2022. We first captured and identified birds using mist nets, obtained fecal samples using non-invasive methods, and recorded the bird's age, sex, fat score, weight, and other associated data. We used an elemental analyzer (EA) and inductively coupled plasma-optical emission spectrometry (ICP-OES) to measure the concentration of 12 elements (C, N, P, Ca, K, Na, S, Si, Fe, Mg, Zn, Al) in 180 fecal samples. We also measured stable isotopes (δ13C, δ15N) to reconstruct short-term diets. We examined which traits best explained elemental concentrations and ratios of waste within and across species. We found that birds vary dramatically in the nutrients they release. Based on principal component analyses, birds show specific nutrient niche spaces that appear to correlate with dietary differences as suggested by isotopic ratios. This was supported by regression analyses with herbivorous birds excreting lower concentrations of N, P, Ca, and Na and higher concentrations of C compared to carnivorous birds. However, adding season as a covariate explained additional variation in C concentrations, with out-migrating (spring) birds excreting less C relative to other nutrients potentially due to higher rates of C sequestration during periods of fat production before migration. Overall, our study highlights that nutrient release by birds varies widely in elemental composition, primarily linked to dietary differences and phenology. These findings can be used to further evaluate avian impacts on ecosystem processes and suggest that birds can play diverse functional roles in nutrient dynamics. Future research may examine the particular importance of migratory birds in the nutrient dynamics of stopover sites and explore greater taxonomic and geographic diversity.
