For the Birds Radio Program: Cooper's Hawk Eye Color
One of the interesting facts I learned when I first studied hawk identification was that you could figure out the age of accipiters—that is, the Goshawk, Cooper’s Hawk, and Sharp-shinned Hawk—by using a combination of plumage and eye color. Birds hatched this year will have immature plumage by the time they leave the nest. This plumage is quite different in color and pattern from adult plumage, which this year’s birds will molt into before next spring. So the feathers tell you if you have a bird less than a year old. Beyond that, eye color in accipiter species ranges from yellow through orange to ruby red. Scientists have long believed that one-year-old birds have adult plumage but yellow eyes, two-year-old birds have light orange eyes, three-year-old birds have deep orange eyes, and birds older than that have red eyes.
But this long-trusted ornithological fact turns out to be not true at all. The latest edition of The Wilson Bulletin has a fascinating article by 7 ornithologists from Wisconsin, British Columbia, North Dakota, and Minnesota. Robert Rosenfield and the other researchers used data from Cooper’s Hawks banded and then recaptured during subsequent years to determine that yes, Cooper’s Hawk eye color does deepen over time, but this does not happen at a constant or consistent rate. Birds from British Columbia developed darker eyes much more rapidly than ones from Wisconsin, and there was a great deal of variation in the rate of change in birds from all three study areas. They also found that females in all areas develop deeper eye colors much more slowly than males, and few females ever get the deep red color that males do.
The researchers also studied mated pairs of birds to determine if Cooper’s Hawks breed assortatively—that is, choosing mates by using an identifiable physical feature to rank potential partners such that the fittest males and the fittest females select one another, moving down to the least fit males and least fit females ending up with one another. Ornithologists had believed that birds with deep red eyes were easily identified as the oldest and thus most experienced breeders, and so would be the choicest mates. If this were true, females with the darkest eyes would tend to mate with males with the darkest eyes, and birds with the lightest eyes would end up with pale-eyed mates. But this study found no evidence that birds with darker eyes were more fit or successful in rearing young than those with paler eyes. And in a recent paper by the two senior authors of this research, they demonstrated that Cooper’s Hawks preferred mates with larger body mass and wing size, and that these characteristics are, indeed, good indicators of overall fitness and reproductive success. Since this study clearly demonstrated that some older birds have much paler eyes than young ones, apparently eye color is far more interesting for humans studying Cooper’s Hawks than the hawks themselves.
When I was a little girl, I thought if a person read enough books, he’d know just about everything, and that you could find the answer to any question if you consulted the right book. It didn’t occur to me until I was in college that we don’t yet know the answers to many questions, and that some answers we thought we knew were really more complicated than we thought, and sometimes even wrong. At a time in our nation’s history when it appears that more and more people yearn for simple black and white answers to every issue, we learn that the real world is ever more, not less, complex, the patterns richer and more varied than we dreamed.