UPDATE: John sent an appropriate Valentine’s Day photo to add to the “eyes” collection below.
A male and a female Phainopepla (Phainopepla nitens):
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We’ll continue tomorrow with David Hughes’s final set of photos from India, but today we have a special Valentine’s Day bird contribution from evolutionary ecologist John Avise. It’s called “Pretty eyes,” and John’s notes and IDs are indented:
Why do many avian species have such beautifully colored eyes? The proximate (mechanistic) answer of course involves various pigments in the iris, often in conjunction with how light is diffracted by the physical structures of cells. But what are the ultimate (evolutionary) explanations for colorful eyes? Could it have to do with mate choice, species recognition, and the operation of sexual selection? Or, in some species, might it involve some form of natural selection such as crypsis (camouflage), or perhaps even aposematic (warning) coloration? Or, in some cases might it just be a non-adaptive byproduct of the body’s need to dispose of breakdown products (such as from bile or hemes)? I don’t have the answers, so I’ll leave it to readers to suggest their own favored evolutionary explanation(s). In the meantime, let’s simply enjoy some of the beauty and variety of avian eyes
Orange eye of the Rock Pigeon (Columba livia):
Bright-red eye of the Western Grebe (Aechmophorus occidentals):
Aquamarine eye of the Double-crested Cormorant (Phalacrocorax auritus):
Bright red eye of the Eared Grebe (Podiceps nigricollis):
Chocolate-brown eye of the Limpkin (Aramus guarauna):
Bright blue eye of the Anhinga (Anhinga anhinga):
Bright red eye of the Phainopepla (Phainopepla nitens):
Orange-red eye of the Sandhill Crane (Grus canadensis):
Bright yellow eye of the Burrowing Owl (Athene cunicularia):
Bright red eye of the Spotted Towhee (Pipilo maculatus):
Orange-brown eye of the California Towhee (Pipilo crissalis):
Pinkish eye of the Tricolored Heron (Egret tricolor):
Light blue eye of a juvenile Cooper’s Hawk (Accipiter cooperi):
Pastel-blue eye of the Little Blue Heron (Egret caerulea):
Orange eye of a juvenile Black-crowned Night Heron (Nycticorax nycticorax):
In some species, such as this drake and hen of the Lesser Scaup (Aythya affinis), the two sexes show different eye colors:
Brown eye in the female Brewer’s Blackbird (Euphagus cyanocephalus):
Yellow eye in the male Brewer’s Blackbird:
Devilish red eye of the Rockhopper Penguin (Eudyptes (chrysocome):
Well, they look like Skittles, so maybe it indicates the flavor?
I think several of them shout: ‘sexual selection’, at least to me (Anhinga, Brewer’s Blackbird, Western Grebe, Phainopepla,Lesser Scaup). Others much less.
Quite a bit depends on their mating system before really concluding to sexual selection, I’d guess.
Based on the drake and hen of the Lesser Scaup, I’d say sexual selection is probably at work in many of these cases. The male has eyes that are bright and contrast with their background. It’s distinct species and sex identity is enhanced. The female’s color is subdued and serves to help hide the duck from predation.
An interesting subject! So my opinion, which is not worth much, is that it could well be for all of the reasons you suggested. I think in some species males have a striking eye color, so sexual selection in those cases. In others it seems consistent with camouflage. And in others it could be no direct selection at all.
Thanks for your beautiful work, John Avise!
I learn something new everyday on WEIT, and I get to ‘fatten my eye’.
Those are outstanding photos.
The Spotted Towhee on red bricks. Fantastic.
Very nice photos, thanks!
Ditto!
Not in birds, I’m afraid, but irrefutable evidence that sexual selection in eye colour works:
https://images.findagrave.com/photos/2018/25/30103583_1516961247.jpg
Beautiful. Wondering if the name Avise pre-destined you (or other family members) to be bird experts.
Very interesting.
Many of these eyes — especially the red ones — seem to have an attention-getting function.
A perhaps parallel concern for me for many years is the way many birds’ eyes are camouflaged or hidden by narrow stripes of color.
The stripes are formed by colored feathers, & the stripes seem to have an adaptive function.
These feathers & stripes are often dark or white, & they seem to incorporate & hide the eyes.
Such photos appear all the time in nature publications
Are there studies that explore these phenomena?
An interesting thing about birds which I never gave much thought, scent, smell is available to only a few species as a means of foraging, food detection, most birds seem heavily dependent on their eyes.
Didn’t take long to find this,
https://www.google.com/amp/s/www.sciencealert.com/birds-can-see-uv-light-now-scientists-can-show-us-what-that-looks-like/amp
This also from the link below.
https://frontiersinzoology.biomedcentral.com/articles/10.1186/s12983-017-0243-8
In birds, only two comparative studies have been published so far to our knowledge, both on passerines. One of them [14] reported an inexplicable higher proportion of bird species with bright irises in Southern Africa (25%) and Australia (35%) versus Canada (6%) and Europe (8%). The other study [5] reported that non-cavity nesting birds are under strong selection to evolve dark eyes, and that eye color was unrelated to parental care. At a general level, there is also the suggestion that stalking predators, whether mammals or birds, tend to have yellow or light-colored irises, whereas predators that run after their prey, and prey-species themselves, tend to be dark-eyed [2]. Despite the limited number of studies that explicitly look at variation in eye colour, they all suggest selection constrains variation within species.
We sure do love the dinosaurs here!