The
color of a bird's eye (usually the color of the iris)
results from both pigments and phenomena such as the
diffraction of light. Avian eye colors range from dark brown
and yellow through red, blue, and green to metallic silver
and gold. In some species, eye color differs between the
sexes, as in bright yellow-eyed male and brown-eyed female
Brewer's Blackbirds. The nearly identical sexes of the
European Starling can be differentiated by the presence of a
yellow ring along the edge of the iris in
females. In many species, eye color
changes as the bird matures and can serve as a means of
determining an individual's age. Although the physiology of
iris pigmentation is poorly understood, changes in color
with age and with season are likely to be under hormonal
control, especially where colors are closely associated with
the sexual cycle. Changes of eye color with age are found in
a wide variety of avian families including the loons,
grebes, ducks, hawks, pheasants, gulls, alcids, woodpeckers,
mimic thrushes, vireos, and blackbirds. Species requiring
more than a year to pass from juvenile to adult plumage
(such as the Bald Eagle and Herring Gull) generally show a
concurrent change in eye color. Some specific examples of
age-related changes are Lesser Scaup and Northern Harrier
(from brown to yellow), Sharp-shinned Hawk (bright yellow to
red), Red-tailed Hawk (yellow to red-brown), American Crow
(blue or blue-gray to brown), Dark-eyed junco (gray or
gray-brown to red-brown), and Common Grackle (brown, turning
paler with age). The evolutionary significance of these
changes is not clear, but in some birds they may serve to
help determine the maturity of potential mates. SEE: The
Color of Birds;
How
Long Can Birds Live? Copyright
® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl
Wheye.