Ecological and evolutionary influences on the elemental composition of birds.

The evolutionary and ecological diversity of animals is often reflected in the elemental composition of their bodies. Despite decades of stoichiometric research, remarkably little is known about the elemental composition of birds, the most diverse group of land vertebrates. This gap limits our understanding of vertebrate body composition and its implications for ecosystem functioning. Here, we report the body stoichiometry (%C, %N, %P, C:N, C:P, N:P) of 32 bird species spanning diverse ecological traits and phylogenetic lineages. Compared to other vertebrates, birds exhibit consistently low phosphorus content, probably reflecting two key flight adaptations: skeletal minimization (i.e. restricted investment in phosphorus-rich bone) and feather production (i.e. investment in phosphorus-poor keratin). Among birds, carbon content is associated with body fat, which has distinct stoichiometry and is known to fluctuate seasonally. Feathers constitute ~25% of a bird's body nitrogen on average, hence variation in feather investment can produce significant differences. Unlike patterns observed in other vertebrates, body size, taxonomy, phylogeny and diet poorly predict bird stoichiometry. Instead, we infer that selective constraints arising from flight (skeletal minimization, feather investment) and phenological cycles (fat storage, feather molting) shape bird stoichiometry. These findings can inform research on avian nutrition, ecology and zoogeochemistry amid global change.