Eamon Callahan Corbett, Robb T. Brumfield, Brant C. Faircloth

Birds display a rainbow of eye colors, but this trait has received relatively little study compared to plumage coloration. Avian eye color variation occurs at all phylogenetic scales: it can be conserved throughout whole families or vary within a single species, yet the evolutionary importance of eye color variation within species and across the avian phylogeny is also understudied. Here, we provide an overview of eye color variation in birds, summarizing the state of knowledge about its causes at three primary levels: mechanistic, genetic, and evolutionary. Mechanistically, we show that common avian iris pigments include melanin and carotenoids, which also play major roles in plumage color, as well as purines and pteridines, which are found as pigments primarily in non-avian taxa. Genetically, we examine known associations between genes and eye color, primarily identified from classical breeding studies and recent genomic work in domesticated species. Finally, from an evolutionary standpoint, we present and discuss several hypotheses explaining the adaptive significance of eye color variation. Many of these suggest bird eye color plays an important role in intraspecific signaling, particularly as an indicator of age or mate quality, although the importance of eye color may differ between species and few evolutionary hypotheses have been directly tested. We suggest that future studies of avian eye color should focus on all three levels, including broad-scale iris pigment analyses across bird species, genome sequencing studies to identify loci associated with eye color variation, and behavioral experiments and comparative phylogenetic analyses to test adaptive hypotheses. By examining these proximate and ultimate causes of eye color variation in birds, we hope that our review will spur future research to understand the ecological and evolutionary significance of this striking avian trait.