Ever since my recent encounter with a snowy owl (January 25, 2022, post), I’ve been trying to find out whether the colors of birds’ eyes, especially yellow irises like the snowy’s, provide functional sight advantages. My rather superficial research has turned up a few interesting theories, but no seemingly conclusive answers.
Bald Eagle
One (highly criticized) theory relating to the sharp-sighted bald eagle is that its semi-transparent yellow irises, compared to darker ones, apparently let more diffused light get to the image-recognizing retina; and, that additional light may have a yellow cast to it that increases focusing sharpness. (Glasses with yellow lenses seem to make images sharper for certain humans with vision problems, perhaps the same effect works on normal bald eagles, according to this theory, which seems to lack scientific support.)
Another theory has to do with the association of eye color with the hunting habits of owls. Those that hunt mostly at night, such as barn and barred owls, have black or dark brown eyes.
Barb Owl
Barred Owl
Those owls with yellow eyes, such as short-eared, great horned, and snowy owls, seem to prefer hunting during the day.
Short-Eared Owl
Great Horned Owl
Snowy Owl
Whether the colors have evolved to enhance hunting success apparently is not known. (All images from the Leighton archives.)