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THIS WEEK at HILTON POND
22-30 April 2003

Installment #170---Visitor #

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THE "EYES" HAVE IT

Some birds--including the White-eyed Vireo (above) and the Red-eyed Vireo (below)--are even named for their easily viewed eye colors, but with the help of a hand lens we see far more detail in these birds' eyes than is otherwise possible. Exceedingly uncommon in the bird world are the ghost-like eyes of the White-eyed Vireo (WEVI); a magnified view shows the iris appears to contain white granular material and that the outer edge is laden with pink blood vessels. This is very different from the iris of the Red-eyed Vireo (REVI), laden with red pigment. Curiously, young WEVIs actually start out with muddy, gray irises that gradually become crystal-clear and white. Juvenile REVIs have irises that are brownish and muddy. The REVI below is a second-year bird hatched in 2002 and banded this week; its eye likely will darken, redden, and clarify further over the coming months.

Indeed, most young birds have irises that are different in hue and clarity than adults of the same species--a phenomenon nicely illustrated in a series of Sharp-shinned Hawk photos (below). Although these are three different birds, they show the gradual shift from light to dark that is typical of some bird species. (A similar change from yellow-orange to red occurs in Eastern Towhees, although some individuals we've retrapped in successive years never demonstrate the phenomenon. In some coastal and southern populations, the iris is white in adult towhees.) We suspect the change in iris color over time provides evidence of a bird's age--important, for example, when a mature female's reproductive potential is enhanced by her selection of an older and therefore healthier and more "worldly wise" father for her offspring.

A bird's iris includes millions of pigment cells. These, in turn, contain a variety of compounds--especially melanin--and other substances such as colored oils and guanine. All these substances work in combination through reflection, refraction, and scattering to give each bird its unique eye color. In albino birds, pigment cells lack pigments entirely and the underlying blood vessels make the iris appear pink. In some bird species, eye color also may be determined by hormonal changes--as when the iris of a Brown Pelican becomes powdery blue during the breeding season.

A bird in the hand reveals another eye-related structure that is nicely adaptive for the avian lifestyle. Like most vertebrates, birds have eyelids that close over the eye--blocking out light when it's time to snooze, or protecting the delicate organ from abrasion and impact. But birds have another protective device--the nictitating membrane--that is seldom noticed. This transparent "third eyelid" rests at the anterior edge of the eyeball and can sweep backward (see Common Grackle above), cleansing and temporarily covering the eye while still allowing a view--albeit milky--of the bird's surroundings. (By the way, the human eye also has a nictitating membrane--it's part of that little pink thing in the corner of your eye nearest your nose. Ours is vestigial and no longer functions as it does among the Aves.)

In birds the nictitating membrane is especially useful because these winged organisms typically spend lots of time flying. In the air, even momentary loss of vision caused by closed eyelids might throw a bird off balance, causing it to plummet earthward. And isn't it great that birds such as falcons--which intentionally dive at high rates of speed--can protect their eyes from the rushing wind simply by closing their nictitating membranes, or that loons and ducks have a clear central lens in the membrane that allows them an undistorted view when swimming underwater?

As typical birds, White-eyed Vireos have nictitating membranes, and a comparison of the top photo and the one at right shows just how different the bird's view of the world must be when--as is shown here--the membrane has nearly finished sweeping back to cover the eye.

All birds have round pupils, except for the Rhyncopidae, which includes the Black Skimmer found in coastal regions; this family has pupils that are cat-like and become narrow vertical slits in bright light. Regardless of the species, it's really hard to see the white of a bird's eye because pupil and iris essentially fill the exposed portion of the avian eyeball.

Curiously, some birds such as Brown Thrashers apparently can "see" even when their eyes are closed, since the eyelid itself is unfeathered, thin, and semi-transparent (left). The thrasher also demonstrates that in birds the bottom eyelid moves upward--opposite of the way the lids work in humans. The various close-up images on this page likewise reveal that just as humans have modified hairs that line the eyelids and filter out dust, birds have short, bristly feathers that serve the same function.

The bird's eye is undeniably one of the most interesting structures visible to bird banders here at Hilton Pond Center, including when we handle Eastern Bluebirds (adult male below). Thus, we're sure you'll understand why we say that within the Aves, the "eyes" have it.

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