Carotenoid-dependent plumage coloration is associated with reduced male care in passerine birds

The immense diversity of plumage coloration exhibited by birds is the result of either pigments deposited in the feathers or microstructural arrangements of feather barbules. Some of the most common pigments are carotenoids, which produce bright yellow, orange, and red colors. Carotenoids differ from other pigments since birds cannot synthesize them de novo and must obtain them from the diet. Carotenoid pigments are usually associated with signaling and sexual selection, although they also have antioxidant properties and play a role in the immune response. Here, we hypothesize that carotenoid-dependent plumage coloration functions as a signal of a male's tendency to invest in offspring care because they play an important role in self-maintenance and may provide key information about individual quality; allowing females to obtain information about a male's tendency to invest in offspring care. Using phylogenetic comparative analyses across 349 passerine birds, we show that species that consume carotenoid-rich foods have more carotenoid-dependent plumage coloration than species with carotenoid-poor diets. In addition, carotenoid-dependent plumage coloration is associated with decreased male investment in offspring care. Our results suggest that investment in carotenoid-dependent plumage coloration trades off against male investment in offspring care and will likely have broad implications for our understanding of the ecological contexts that facilitate various evolutionary processes, such as sexual selection and signaling associated with plumage colors. Among songbirds, males with bright red, yellow, or orange plumage coloration invest less into parental care of offspring. To show this, we analyzed the relationship between carotenoid-dependent plumage coloration (bright red, yellow, or orange colors) in males and male investment into offspring care across 349 song-bird species. Males with bright red, yellow, or orange plumage coloration were found to provide a lower share of parental care of offspring compared with males that do not have such coloration.