Bird Eye Color

Summary: Have you ever noticed how many different eye colors birds have? Join Kiersten and Cheryl as they delve into this understudied topic.

 

For our hearing impaired listeners, a transcript of this podcast follows the show notes on Podbean.

 

Show Notes:

“Bird Eye Color” A Rainbow of Variation, a Spectrum of Explanations,” by Eamon C. Corbett, Robb T. Brumfield, Brant C. Faircloth. https://ecoevorxiv.org/repository/object/3682/download/7316

“Bird Eyes Come In an Amazing Array of Colors – but Why is a Mystery,” by Meghan Bartels; Audubon February 2023. www.audubon.org

Our email address, please reach out with comments, questions, or suggestions:  thefeathereddesert@gmail.com

 

Kiersten: Intro: Eye color in birds is often something we might overlook. But it’s incredibly fascinating. Bird’s eyes come in a variety of colors and can even vary throughout one individual’s life. Today Cheryl and I are going to talk about eye color in birds.

 

Cheryl: We all know how colorful bird’s feathers appear. They can be blue, red, yellow, even green. This is one of the reasons birds have been studied by scientists for so long and one of the main reasons we want to attract birds to our backyards. We want so see those bright, fabulous colors. Scientists want to know what’s up with all the colorful feathers, and while this is fascinating and important research, it has overshadowed the other amazing colorful characteristic of birds: their eyes.

            Here in the southwest, the Curve-billed Thrasher, is an example of bird with a brightly colored eye. Adults have bright orange eyes. It’s an iconic characteristic and a great way to distinguish them from other types of thrashers.

            Birds’ eyes come in many different colors from dark-brown to light-brown to reds, oranges, yellows, greens, blues, and even white. Within these colors, a range of hues exist such as the turquoise of the Double-crested cormorant, the emerald green of the Black-and-red Broadbill, the crimson red of the Bronzed cowbird, the sunset orange of the Painted buttonquail, and the school bus yellow of the Short-eared owl.

            Birds even have multi-colored irises. Our Curved-billed Thrasher’s orange eye is actually a combination of an outer ring of orange with an inner ring of yellow to red near the pupil. The underappreciated Rock Pigeon actually has a beautiful eye with a ring of red on the edge and a ring of yellow around the pupil. The Greater Roadrunner, another of our southwest residents, has an eye with rings of brown and yellow around the pupil. Most of the birds with multi-colored iris have symmetrical coloration, but a few have asymmetrical coloration.

            The Bank Cormorant’s eye is an earthy orange on top while the bottom looks like a turquoise stone. Looking at this bird’s eye is like looking at an Arizona sunset. Some female oystercatchers have dark eye flecks on only one side of the pupil giving their eye a keyhole appearance.

 

Kiersten: It is amazing the colors that are found in the avian world, but the light bright colors are less prevalent than the darker colors. A study released early this year, in 2023, has compiled information previously researched about eye color and they noticed some trends.

            It appears that more birds have darker eye color, such as brown and black, than light eye color. Birds, as a whole, have darker eyes on average than other vertebrate groups including mammals, reptiles, and amphibians. Looking at passerines, such our songbirds, 64% had black to dark brown eyes, 19% had light brown to red eyes, and 17% had the lightest eye colors including yellow, orange, white, pink, blue, and light gray.

            Non-passerine birds, such as hawks, owls, ducks and other water birds, have lighter eyes than passerines. 40% were dark, 22% were the intermediate light browns and red, while 38% were light. The dark versus light hues found in birds do tend to follow phylogenetic lines. Birds in related families tend to have the same level of eye color. They may not always have the same eye color exactly but they are often categorized the same as dark, medium, or light. What we’ve talked about so far is differences of eye color between species but there are differences in eye color within species.

 

Cheryl: There are typically three reasons eye color will vary within a species. Number 1: Some birds are born with a darker eye color that will lighten as they age. For example, our Curve-billed Thrasher is born with brown eyes that change to orange only when they are adults. Spotted towhees have dark brown eyes as juveniles that turn red when they are adults. Osprey chicks are born with orange eyes that lighten to yellow when they are adults.

Number 2: Twenty-four species of birds have been shown to have differences in eye color between sexes. Essentially males and females of the same species will have different colored eyes. Common grackles are a good example of this. The males have bright yellow eyes, while the females have brown eyes. Brewer’s blackbird females have dark brown eyes while the males have a bright yellow. In a reversal, Saddle-billed Stork males have dark brown eyes while the females have bright yellow eyes.

Number 3: Eye color can vary seasonally. A small handful of birds that we currently know about will change eye color during breeding season. The Brown Pelican male’s eyes will change from a brown color to a light blue at the beginning of breeding season and remain blue until incubation begins. The male Great Blue Heron’s eyes will turn red during breeding season and be yellow the rest of the year. Female chickens eye color may change when they are laying eggs.

            There are a few birds that are able to change their eye color based on their mood. They can expand or contract the blood vessels in their eyes to change the color of the iris. We actually have a bird in our own backyard that can do this, the Inca Dove.

            For those of you who are cross-country birders, you may be familiar with one last reason that the same species of birds can have varied eye color, population differences. Species that have a large habitat range can develop differences in eye colored based on the eye color that is most popular in a specific region. For example, a House Finch from Arizona might have a slightly different eye color from a House Finch born in New Jersey. They will most likely be in the same range of color but they can vary.

 

Kiersten: Now that we know what kind of colors exist in birds’ eyes, let’s look at how these colors are created. In humans’ eyes different levels of melanin create our eye color. Melanin is also involved in creating color in bird feathers, for more on this please check out our previous podcast on feathers. If you are thinking melanin is involved in creating bird eye color, you’re partially right!

            There are a variety of pigments, blood vessel dilation, colorless oil droplets, collagen fibers, and other structural features that create the colors that we see in bird eyes. According to the 2023 research paper, color created by chemical pigments are created by living cells called chromatophores which is unique to avian eyes.

To further amaze us, we cannot assume that one bird with a red eye created by the carotenoid pigment means that all birds with red eyes have carotenoids making their eyes red. For example, Canvasbacks, Red-eyed Vireos, and Bronzed Cowbirds all have red eyes but they are all produced by different means. Canvasbacks red eyes are created by carotenoids, Red-eyed Vireos are created by another pigment called pteridines, and the red eyes of Bronzed Cowbirds are caused by the dilation of blood vessels in the eye.

The known mechanical contributors of bird eye color are carotenoids, melanins, purines, pteridines which are all pigments, and blood vessels, collagen fibers, oil droplets, and cholesterols. Birds can have reflective structures such as a crystalline pteridine or purine, colorless oil droplets, or collagen bundle in the eye that often creates the lighter colors. To create, color birds can have one or a combination of any of these. Species with only pigments present in the eye typically have a duller eye color such as the Northern Flicker and the Cactus Wren. Now, I’m not saying their eyes aren’t a nice color or even a pretty color, I mean more like the difference between a shiny new penny versus an older well-worn penny.  Birds with only reflective structures in the eye have a white iris, like the Acorn Woodpecker.

            Generally speaking, birds with darker eyes have more melanins while birds with lighter eyes have more purines and pteridines. Birds with bright, almost sparkling eyes have reflective structures such as crystalline purines or pteridines, collagen bundles or oil in the eye. The next question is why do birds have so many different eye colors?

 

Cheryl: The answer is….. we don’t really know. Very little research has actually been done on eye color in birds. What we can do is generalize about eye color based on other attributes of birds. We know that birds that molt into adult colors do not breed until they have their adult color pattern, that could be a reason for juvenile change of eye color as well. The change of eye color may indicate readiness to breed. The same can be said for changing colors before breeding season. A male with a good head of feathers can indicate a healthy male and attract female attention. This might be the same reason for the change of eye color before breeding season begins.

            Other reasons could be survival. The birds with a particular eye color survive best and they are the ones to pass down their genetics. Maybe the eye color allows them to camouflage better than other individuals. Maybe it’s based on how they obtain food. Foragers versus hunters? Is there a pattern to who has which eye color? What about their habitat? We do know that birds that live in sunnier places have more melanin in their feathers because it makes them last longer. Maybe this is also true for their eye color? Maybe the melanin protects their eyes. Does the eye color have anything to do with communication between individuals of the same species? We know that wing postures, feather color, and sound are all important in communication, maybe eye color is as well. Or does none of this have anything to do with eye  color at all?

            Sadly, we don’t have the answers because no long-term research has been done on bird eye color. It is definitely a topic that needs much more attention in the future.

 

Kiersten: One of the things we can do as citizen scientists to help this research along is take pictures! Especially those of you with the patience for wild bird photography. Taking pictures of birds eyes with your telephoto lenses and uploading them to eBird will help these scientists move forward in their research. 

Closing: What we’ve talked about in this podcast probably seems like a lot of information, but this research was only done on a small portion of bird families. We need much more research done concerning bird eye color. And, we don’t know yet, but this could be the next big epiphany we have in animal behavior. What we know today is terribly fascinating, but I think many more amazing discoveries await us in the future.