Why do birds have different eye colours?

No one would consider cormorants to be the most colourful bird family – until you look at their eyes. Their striking irises range from turquoise (Double-crested), to sapphire blue (Brandt’s), forest green (European Shag), deep red (Long-tailed), and even a bicolored orange-and-teal sunset pattern (Bank). But even that impressive diversity is only a fraction of the array of eye colours that can be found in birds. While feathers receive most of the attention, bird eyes are equally dazzling in their variety: as illustrated by the collage below.

When considering this incredible variation, the obvious question to ask is “why?” But there are many different answers, depending on how deeply or broadly we look. We could say that a bird has red eyes because it has red pigment in its irises. Or, because it has a particular variant of an “eye colour gene”. Or, because it uses its bright eyes as a display signal in the dark forest understory. Each of these explanations focuses on a different level of causation: from mechanistic, to genetic, to evolutionary. In our recent review, we surveyed over 100 years of research and found that, though some broad patterns have emerged, much work still remains to get a full understanding of the causes of bird eye colour (Corbett et al. 2023).

In bird feathers, the predominant pigments are melanins (black, brown, grey, rufous), and carotenoids (red, yellow, orange). Both are also present in bird eyes, but two other types of pigments – pteridines and purines – account for many brightly coloured bird eyes. A variety of other factors, including blood vessels, collagen fibres, cholesterol crystals, lipid droplets, and structural elements, can also contribute to eye colour. Complicating matters further, the same eye colours can be produced by a variety of different mechanisms, so we can’t make assumptions about the elements involved in a given species without conducting chemical analyses or examining the iris under a microscope. We do have information on iris pigments and structures for around 180 species, but 98% of species and 79% of families have been entirely unstudied so there is still much to uncover.

We know even less about the genes involved in bird eye colour. Starting in the early 20th century, experimental crosses of domestic chickens and pigeons have identified some patterns of inheritance of eye colours, but the first specific eye colour gene in birds was identified only in the past few years. Three independent research groups studying domestic pigeons found that a mutation in a gene called SLC2A11B results in a lack of yellow pteridine pigment, creating what pigeon breeders call “pearl” eyes. So far, however, no similar genes have been identified in wild bird populations.

Studying pigments and genes can help us understand how bird eyes get their colour, but what advantages do colourful eyes provide in the first place?

One possibility is that they affect birds’ survival, through effects on their vision or camouflage. It’s not certain that the visible colour of the iris would affect how light reaches the retina, because there is a lot of light-absorbing pigment deeper in the eye that is not externally visible. Still, the idea that eye colour could influence visual acuity, perhaps by reducing glare, has yet to be tested. On the other hand, studies of owls and cavity-nesting birds have raised the possibility that eye colour could be associated with camouflage, with more vulnerable species like day-roosting owls and open-nesting songbirds having darker, less conspicuous eyes.

In many birds, signalling may play a greater role in the evolution of eye colour. The vivid eye colours of some birds-of-paradise, bowerbirds, and cormorants seem to be under the influence of sexual selection, just like colourful feathers. Since eye colour varies within species, it could serve as a marker of age, sex, or mate quality. Or, it could simply be an announcement of an individual’s presence: one experiment in jackdaws found that their bright eyes dissuaded rivals from approaching their nest cavities.

The study of bird eye colour variation has the potential for many new discoveries. Paying greater attention to eye colour variation, taking standardised eye photos during bird ringing and specimen collection, and harnessing the potential of photo databases like the Macaulay Library will give a more complete understanding of variation within and across species. Conducting High Performance Liquid Chromatography and other chemical analyses in a range of species will uncover the pigments and structures that underlie that variation. Whole-genome sequencing in polymorphic populations and hybrid zones will help identify the first eye colour genes in wild birds. And comparative phylogenetic analyses and behavioural experiments will identify and test hypotheses about the adaptive significance and evolutionary origin of bird eye colours. It will take many teams of researchers using disparate approaches, but the combined result of these efforts will be a much deeper understanding of the rainbow of colours found in bird eyes.