Surf Scoter
Surf Scoter
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Of humanity, it has been said that the eyes are the windows into the soul, and our eyes certainly express an array of moods and emotions. When people fake a smile, however, onlookers can often tell because the corners of the smiler’s eyes do not crinkle up. Birds have no concerns about smiling, but their large, powerful eyes are essential gear for survival.

Vision is the most vital sensory system for birds, and avian visual acuity is estimated to be 2 to 8 times better than humans’. In relative size comparisons to human heads, birds’ eyes are huge. Eyes of eagles and the larger owl species are approximately people-sized. Ostrich eyeballs, with their longish curled eyelashes, are nearly double the size of our own. Being so large, birds’ eyes are fixed in their sockets, meaning that birds must twist or rotate their heads to peer around or glance skyward.

The precise positioning of birds’ eyes reveals a lot about their life status as either a prey or predator species. With eyes located on the sides of their head, prey species, like sparrows, have a wide peripheral field of view that is useful in detecting predators. In contrast, the eyes of predators are positioned closer to the front of the head, allowing binocular vision and increased depth perception for estimating distances while hunting.

Researchers have documented a unique neurological configuration that allows some birds to sleep with one eye open as other portions of the brain rest and sleep. During a state of “unihemispheric slow-wave sleep,” these birds can maintain vigilance for approaching predators while getting a bit of shut-eye. Other visual advantages include birds’ ability to view light within the ultraviolet spectrum and the superior night vision capacity of many species.

The two eyelids function rather differently in various species: owls shut and blink their eyes by moving the top eyelid down (as we do); other birds move the bottom lid up to shut the eye.

As with your cat or dog, birds also have a third eyelid called the nictitating membrane, a thin semitransparent eyelid that moves horizontally across the eye to protect it from injury or sweep away debris. Raptors use the eyelid to protect their eyes while feeding lunging nestlings. Cormorants and some diving birds have a small transparent central window in the nictitating eyelid that adjusts or corrects the underwater scene.

Concentrating mainly on shapes and feather details of birds, we may neglect to notice their eye color. The majority of birds have brown-toned eyes, but diversified color hues run the gamut. In certain species, eye color evolves as birds mature: the “baby blue” eyes of nestling crows and ravens assume a dark brown color in adulthood. The eye color of woodland accipiters, like the Cooper’s and Sharp-shinned Hawks, transitions from medium yellow to deep orange as the years pass. Some species, like closely related Red-eyed and White-eyed Vireos, are directly named for their distinctive eye shades.

I’ve included some up-close field photos that illustrate eye color variations in several species. Did you realize that Double-crested Cormorants have deep turquoise eyes? Did you know that the lemon yellow eyes of Snowy Owls are almond shaped to reduce glare from Arctic ice and snow cover? And how about those intense white peepers of the wintering male Surf Scoters!