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,
environment, 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
origin 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.