Feathers steal the show, but there’s another feature every bird has that also comes in a rainbow of colors: their eyes. And, somewhat surprisingly, scientists know very little about the hows and whys behind all this variety.
A recent study shines a spotlight on the diversity of avian eye color while calling on scientists to further investigate the incredible array of hues: red, yellow, blue, pink, white. “When we think about bird coloration, we’re almost always thinking about feather coloration: A cardinal is red, a Blue Jay is blue,” says Eamon Corbett, an ornithology Ph.D. student at Louisiana State University and lead author on the new study. “That’s the color of the feathers. But in many cases, they also have these really brightly colored or variably colored eyes,” he says.
And, like feathers, dramatic eyes can be found in even the most common species. The eyes of the underappreciated Rock Pigeon fade from orange at the edge to yellow around the pupil, while a juvenile Osprey’s orange eyes turn yellow at maturity. Brown Pelicans’ brown peepers lighten to blue during the breeding season while herons’ eyes can redden. Where male Wood Ducks sport red eyes, females show dark brown.
To better understand what we do know about avian eye color, Corbett and his coauthors set out to trawl through a century of research, gathering together the tidbits scientists have discovered over the years into one comprehensive paper. In particular, the team focused on three topics: the pigments and structures birds use to create different eye colors, the genetics underlying those diverse hues, and the evolutionary function this variation may serve.
The answers to these questions aren’t necessarily the same for all birds—or even species with similarly colored orbs. For example, Corbett recalled his surprise upon looking into the factors behind three different red-eyed birds. “Canvasback, Red-eyed Vireo, and Bronzed Cowbird are all fairly common North American species with bright red eyes, but the chemical mechanism that creates those red eyes are completely different,” Corbett says. The stark red of a cowbird’s eyes comes from unusually large blood vessels, whereas the Canvasbacks and vireos depend on two completely different types of pigment compounds. “You would never know that just from looking at them, they all look pretty bright red,” he says.
There are a few reasons why eyes have not drawn the same kind of scientific attention as feathers, Corbett and others say. In particular, while feathers can be preserved with relative ease in natural history collections, a bird’s eyes are discarded. “They’re just little dead soldiers laying on their back with cotton for eyes, and I wonder if that kind of makes us not think about how much eye variation there really is,” Shannon Hackett, the Richard and Jill Chaifetz Associate Curator of Birds at the Field Museum in Chicago, says.
At best, scientists poring through study specimens can consult tags on which collectors wrote their impressions of eyes, but Corbett noted that these descriptions can be remarkably idiosyncratic. Although most of his own research dug into the work of other scientists, he recalled stopping by a natural history collection and flipping through 34 tags for a single species that used 24 different color terms. (Hackett noted that scientists at the Field Museum actually discovered a new species in Africa by investigating a former curator’s notes about differing eye colors in one drawer’s birds.)
For scientists seeking to understand how an unusual trait benefits an animal, conducting behavioral experiments is key, but eyes provide a challenge. For feathers, these experiments can involve painting colorful patches black or applying sunblock to feathers that reflect ultraviolet light. But there’s no easy equivalent for eyes, Hackett says. “How do you go about changing the eye color of birds, little tiny bird contact lenses?”
While scientists haven’t traditionally prioritized eye color as a research focus, there is one group that can’t ignore bird eyes: photographers. Sirena Lao, an environmental education specialist and biologist at the San Francisco Bay Bird Observatory, points out that attention to a bird’s eyes is key to keep an image in focus and engaging.
Because of this, both Corbett and Hackett suggest that birders and particularly photographers will be a key resource for scientists attempting to investigate eye color more deeply. The topic is so vast that scientists will need guidance on which species to study first. “It’ll be eBird with all of those photos that will give people the starting point of where to look,” Hackett says.
Lao recommends that birders who want to pay more attention to avian eyes themselves start with larger, easier-to-see species like birds of prey, herons, and egrets. The latter two, coincidentally, are in the family with the highest prevalence of light eyes.
Corbett has his own recommendation. “The North American cormorants I would say have the coolest eyes of any North American species,” he says. “I think a lot of birders, when they first see a cormorant really close up, that’s one of the things that really jumps out, this black bird that has incredible turquoise eyes. And we don’t really know why.”
The Double-crested Cormorant’s bright pop of aqua is only one of the many scientific mysteries Corbett and his colleagues found as they dug into avian eyes while writing the paper, which he says he hopes inspires ornithologists and birders alike to consider species eye color more closely.
For scientists in particular, there are lots of opportunities for better understanding eye coloration in birds. Hackett says that in her own work, she’d love to dig into genomic data to understand where eye colors are coming from, and she hopes the research inspires natural history museums to photograph eyes and other features that they can’t preserve. Other avenues might include looking into the role sexual selection may play in eye color, seeing how eye colors correlate with habitat, and determining whether parasites and microbes might influence pigments.
“It takes you to a new place,” Hackett says of the research. “I think people are going to pay very close attention to this; I’m not going to be the only one who has a whole bunch of ideas that come from it.”