Although most birds have dull beaks, some species have evolved brightly red, yellow or orange coloration in this trait. In a recent study, I compared beak color in over 1600 species of birds to test whether bright bill color evolved as a signal to other birds. I found that bright beak coloration co-evolved with traits related to social competition, suggesting that bill color is a signal used in aggressive interactions. Interestingly, bill color was not related to any measures of sexual selection, and so it probably did not evolve to attract mates.

Dey CJ, Valcu M, Kempenaers B & Dale J. 2015. Carotenoid-based bill coloration functions as a social, not sexual, signal in songbirds (Aves: Passeriformes). Journal of Evolutionary Biology 28: 250-258