For the Birds Radio Program: Loon Eye Color
For years, Laura has been reading, and reporting, that loons have red eyes to help them see underwater. Turns out this isn’t quite true.
When I went down to an ornithological meeting in Oklahoma last month, one of the biggest thrills was meeting Judith McIntyre in person. Judy’s the world authority on Common Loons. She wrote the definitive book on the loon for the University of Minnesota Press and has had popular articles in National Geographic and National Wildlife. I went out to dinner with her one night just to get the real scoop on why loons have red eyes.
Ever since I stared boning up on loons, I’ve been reading so-called experts claim that the reason for the red eyes is to make it easier for loons to see under water. Loons obviously have excellent underwater vision, and can chase and catch fish pretty far beneath the surface. But although at least one loon has been caught in a fisherman’s net set 800 feet below the surface, loons normally don’t dive that deep. Unless they’re darned hungry they actually stay pretty near the water’s surface where the light is best. There’s no evidence that eiders, scoters, and other diving ducks with more dully-colored eyes can’t see just as well under water, and another duck with red eyes, the Wood Duck, doesn’t dive at all.
Another novel theory for explaining loon eye color is that the red eyes help filter out excessive brightness on sun-dappled water, sort of like avian sunglasses. People who expound this theory note that the Red-eyed Vireo and the Sharp-shinned Hawk are also exposed to bright light in their treetop homes, and that these two species also have red eyes. These guys remind me of Mark Twain’s description of scientists who get “such wholesome returns of conjecture out of such a trifling investment of fact.”
Neither theory made sense to me. I was pretty sure the red color was on the iris rather than the lens or retina, and it didn’t make sense that a red iris would have any more effect than brown or blue irides. But I felt uncomfortable arguing about it with the Minnesota DNR and Project LoonWatch and all the other experts that report that the red eye color helps with underwater vision. I knew Judy McIntyre had handled loons and even kept one in her bathtub one winter, so if anyone knew exactly what part of the eye was red, it would be she.
Sure enough, the red color in loon eyes is strictly limited to the iris. Judith McIntyre also pointed out that young loons have dull-colored eyes, even though when they’re first learning to catch fish they obviously require excellent vision. If the red color were truly an aid to vision, the young would certainly have red eyes. She also pointed out that adult loons’ eyes also grow dull-colored at the end of the breeding season, even though they will spend the winter fishing under water in the ocean. Again, the theory that the red color aids in underwater vision just doesn’t hold water.
I asked her about the avian sunglasses theory, and she just laughed.
So why do loons have red eyes? Judith McIntyre believes that it is a visual display rather like the epaulets of Red-winged Blackbirds. Now that makes sense to me. Since vision in most birds is quite similar to human vision, something as eye-catching as a ruby-red eye would certainly attract attention from other loons as well as humans.
So now when Northland visitors ask you why loons have red eyes, you’ll know the right answer.