Natural and sexual selection drive the evolution of carotenoid-based bare-part ornamentation in shorebirds

Carotenoid-based plumage ornaments in birds have long been acknowledged to yield honest signal of individual quality and play a role in intra- and inter-sexual interactions. However, much less scientific attention has been devoted to the role and evolution of non-plumage (bare-part) avian ornamentation. Here, we aimed to investigate processes that shaped the evolution of carotenoid-based ornamentation of bare parts (bill and legs) in Charadrii and Scolopaci shorebirds. Our phylogenetically informed comparative analysis across all extant shorebirds revealed that both natural and sexual selection contributed to the evolution of carotenoid-based bare-part ornamentation. We found evolutionary associations between carotenoid pigmentation and habitat variation, climate, and body size, advocating for the importance of ecological, environmental, and life history traits. Sexual selection (mating system bias and sexual size dimorphism) was also identified as an important driver in the evolution of carotenoid-based ornamentation. Underrepresentation of combinations of carotenoid- and melanin-based ornaments suggested overlapping functions of ornamental traits with different developmental origins or excessive costs associated with their simultaneous expression. Our study adds to the understanding of the complex and multi-faceted processes responsible for the evolution of remarkably diverse avian ornamentation.