Evaluating the roles of signaling and camouflage in the evolution of iris color in Tyranni passerines

Iris color is a conspicuous and diverse trait across animals, but its evolutionary drivers are poorly understood. In over 1,000 species of Tyranni passerines, we tested hypotheses relating iris color to signaling and camouflage, its main putative functions. We expected that, if playing a role in signaling, brighter irises would be positively associated with the strength of social and sexual selection as well as signal conspicuousness, i.e., larger relative eye sizes and contrasting, darker plumage colors around the eyes (contrasting local backgrounds). If aiding in camouflage, darker irises would be associated with higher vulnerability to predation, darker habitats, larger relative eye sizes, and matching, darker plumage colors around the eyes (matching local backgrounds). In support of the signaling hypothesis, we found that brighter irises are associated with darker plumage around the eyes, larger body sizes, territoriality, and sexual size dimorphism. In support of the camouflage hypothesis, we found that darker irises are associated with higher vulnerability to predation (exposed nests, migratory behavior, and terrestrial lifestyles) and larger relative eye sizes, which are associated with darker habitats. Our findings suggest that bright irises are social/sexual signals mostly associated with species under weaker selection favoring camouflage, which has implications for sensory ecology and macroevolutionary patterns of visual signaling.