I’m finally able to fulfill a casual commitment made over two years ago and the delay wasn’t (just) due to procrastination. Turns out, we know very little about the reasons behind the coloration of bird eyes. Additionally, scientists have no standardized method of documenting and tracking bird eye colors.
In a 2021 Daily Bucket comment I said, “I’m going to search for info on bird eye colors. Seems that the dramatic red eyes are not just among water birds … Some birds have different eye colors for males and females. Others change eye color as the bird ages, while some have different eye colors during breeding season. I’ll be sure to share the info I find.”
I followed through right away, Google searching for info, finding nothing, searching again with different search terms and still finding zip. I abandoned generic Google and searched Scholar, but no matter which database or term was searched, the responses offered little about bird eye color but abundant variations on bird eye designs on butterfly wings, how birds discern color and use that info, and even birds-eye views of non-avian topics.
I gave up but the science devas didn’t and answered my query 2.5 years later, last week, when information for my long promised story finally fell into my lap(top) while I scrolled through Twitter. I saw a thread from Eamon Corbett, an ornithology Ph.D. student at Louisiana State University, about his bird eye color review paper, including photos that illustrate some of the bizarre eye colors: The Mechanistic, genetic, and evolutionary causes of bird eye colour variations. This motivated me to do another search and I discovered that, earlier this year, Audubon (notoriously anti-union bird conservation NGO that recently chose to keep the name of a racist enslaver naturalist) had written about Corbett’s study, Bird Eyes Come In an Amazing Array of Colors—but Why Is a Mystery, but I missed them both because I gave up.
The Audubon article includes commentary from Corbett explaining what piqued his interest and what the study found. All quoted text is from that article, but I added the bold.
“When we think about bird coloration, we’re almost always thinking about feather coloration: A cardinal is red, a Blue Jay is blue … That’s the color of the feathers. But in many cases, they also have these really brightly colored or variably colored eyes … 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 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.
Here’s the video — set it to begin at 2:25, the eye color change moment.
For most of these tweet photos, the eye color is best viewed by clicking through to the Macaulay Library source image (just click on the photo in the tweet to bypass Twitter and go straight to Macaulay).
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.
ESPECIALLY CLICK THRU TO SEE THIS BIRD!! The eyes are amazing, but it was those sweet little blue feet that totally captured me.
Another fascinating outcome of Corbett’s work is the recognition that crowd sourced images from bird photographers can be an important source of information for researchers. Bird specimens stored in collections are not useful for eye color studies because, as Shannon Hackett, Curator at Chicago’s Field Museum observes, “They’re just little dead soldiers laying on their back with cotton for eyes...’“ The tag information accompanying each specimen doesn’t help, either, because it relies on the collector’s determination of color that, as Corbett notes, “‘can be idiosyncratic’ ... he recalled stopping by a natural history collection and flipping through 34 tags for a single species that used 24 different color terms.”
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.
The research opportunities are myriad, such as studies of genomic data, the role of sexual selection, whether eye color coordinates with habitat, and how parasites and microbes influence pigmentation. In addition to the photographs submitted by birders and others, perhaps natural history museums will begin to photograph eyes and other features that cannot be preserved in specimens.
Whattaya say birder photogs, ready to add your observations to the bird eye color database via eBird? In the comments, show us the array of eye colors in the birds you’ve photographed.
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