Red-eyed vireo
Red-eyed vireo
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As bird-watchers, we tend to focus mainly on evident field marks to identify or study any certain bird. For instance, an adult bald eagle reveals its gleaming white head and tail and an American robin’s rufous breast is recognizable from a considerable distance. With closer inspection, we detect other distinguishing details such as feather patterns, bill shape and eye color. Eye color, you say? Yes, sometimes eye color can provide diagnostic information. And, although it may be tough for us to discern, eye color is an important element for birds themselves in attracting and distinguishing a mate.

Birds possess a wide spectrum of eye colors that is related to natural pigments. As with human physical development, eye color in birds changes with age and maturity level. The blue eyes of raven chicks, for example, transition to dark brown within a few months. In a reverse scenario, common grackle hatchlings start off life with brown eyes that eventually become light yellow in adulthood. The yellow- colored iris of young Cooper’s hawks passes through various shades of orange before becoming red.

Hormonal changes at breeding time also cause shifts in eye color.

Let’s consider some specific birds with red-colored eyes, a circumstance linked to a natural chemical pigment called carotene. Carotenes are responsible for the orange color of carrots (the namesake for this substance) and the orange hues in dry foliage. Numbers of species, including black-billed cuckoo and killdeer, also have red eye-rings — a bare circle of skin around the eye that turns brightly colored on many shorebirds, gulls and other birds in breeding season.

An aptly named species, the red-eyed vireo is the only American bird with “red eye” in its name. This vireo is one of the most numerous songbirds in northeastern North America but spends the bulk of its time in tall summer shade trees, where it maintains an incessant rhythmic sing-song throughout the hottest days: “Here I am, where are you?” Locating this relentless songster amidst thick layers of high foliage requires some persistence and perhaps a bit of luck. If this bird happens to descend from the treetops, it will appear olive-green, with clean white belly, strong head pattern and broad white eyebrow bordered by blackish lines. Its red eye is the kicker.

The dramatic ruby-red eyeball of adult black-crowned night-herons is a striking component of their identity. This stocky heron’s overall gray body is offset by its black cap and black-and-white belly. Crouched with its neck tucked to ambush prey or slowly stalking the edge of a pond, the night-heron hunts nocturnally for fish, frogs and other prey. Occasionally specializing on eggs and young birds, they can raise havoc at tern colonies. Studies suggest that night-herons may feed nocturnally due to competition or domination by other herons and egrets.

Eye color was a major identity factor in October 2010 when an observant Rockland feeder-watcher discovered a rare male bronzed cowbird among a mixed flock of migrant blackbirds in his yard. Larger-headed than the similar brown-headed cowbirds of northern regions, bronzed cowbirds are normally found in the southwestern U.S. and Mexico. The deep glossy blue glaze on the bronzed male’s wings and body helped to distinguish it from brown-headed cowbirds. There was a more defining field mark, however: both sexes of bronzed cowbirds have deeply red eyes.