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 that 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 the self-maintenance and may provide key information about individual quality; allowing females to obtain information about a males’ tendency to invest in offspring care. Using phylogenetic comparative analyses across 350 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 a decreased male investment in offspring care. Our results suggest investment into carotenoid-dependent plumage coloration trades off against male investment into offspring care and will likely have broad implications for our understanding of the ecological contexts that facilitate various evolutionary processes such as sexual selection or signaling associated with plumage colors.
