Diet, phenology, and body size shape nutrient release by songbirds

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 analyzed 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 δ15N 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.