Bird Eye Color: A Rainbow of Variation, a Spectrum of Explanations

Abstract

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.

The difficult challenge of accurately recording iris colour in birds revealed through the lens of specimens of Australia's Spinifex Pigeon (Geophaps plumifera)

“…The diversity of colour and patterning of avian irides is well illustrated in Corbett et al . (2022; their fig. 1).…”

“…Not all avian eyes show a pupil and a monochromatic, peripupillary iris. The diversity of colour and patterning of avian irides is well illustrated in Corbett et al (2022; their fig. 1).…”

“…Instead, one may need to address variation at the molecular levels of the pigments and genes involved in generating iris colour, and across the entire geographical range not just in the zone of particular interest. Burns et al (2017) and Corbett et al (2022) have reviewed research into these molecular dimensions to the pigments in avian eyes.…”

“…2020, Corbett et al . 2022). Increasing the accuracy of how these colours are recorded will help document future, unexpected changes, which may be due to broad environmental change or even to pollutants (see Bortolotti et al .…”

“…That no rigorous quantitative method for the description of subtle or gross variations in iris colour has been adopted by ornithologists (see, for example, that proposed by Wood & Wood (1972)) and that today's literature still uses essentially subjective descriptions of iris colour all attest to the intransigence of the problem (see Wood & Wood 1972, Polakowski et al 2020, Corbett et al 2022. Increasing the accuracy of how these colours are recorded will help document future, unexpected changes, which may be due to broad environmental change or even to pollutants (see Bortolotti et al 2003).…”