The Web of Life (101-1) Bird Sight


Birds are super visual. They have excellent color vision and good visual acuity. ~ American ethologist Tricia Rubi

Flight is mentally demanding. There is a continual flood of visual information that comes in during flight, integrated into a memory-based mapping system.

The mental processing demands of flight are heightened by the fact that birds have excellent eyesight. This is partly achieved by exceptionally large eyes. A bird’s eye may be larger than a much bigger mammal. A 200-gram owl has a larger set of peepers than a 100-kg human.

The eye of the ostrich is the largest of any terrestrial vertebrate. It approaches the upper limit of optical effectiveness in a biological structure.

Predatory birds generally have relatively larger eyes than other birds. Because a larger eye offers better resolution, a hawk can see a sparrow at a much greater distance than a sparrow can see another bird its own size.

How a bird sees depends on the position of its eyes and the shape of its head. Barn owls have flat faces and forward-looking eyes, yielding very wide binocular vision. By contrast, seabirds such as terns, and most passerines, have a narrow field of binocular vision, but impressively wide peripheral vision. Other birds, such as parrots, with eyes toward the front of round heads, have both broad binocular vision and wide peripheral vision.

The fovea is the portion of the eye with sharpest vision, as it has the highest concentration of photoreceptors. Most animals, including humans, have a single fovea in each eye.

Falcons and other birds of prey have 2 foveae per eye, affording greater visual acuity. Hummingbirds – who hunt flowers – move so fast that they too have 2 foveae, to match their bodily speed with vision ability.

1 fovea, as with other animals, provides good eyesight up close. The 2nd fovea acts as a telephoto lens, providing excellent distance vision.

Bird fovea are adapted to their lifestyle. Many seabirds have fovea that help detect horizons.

Bird eyes have a structure that other animals lack: a pecten oculi. This comb-like pecten sticks out like a large finger from the rear of the eye.

While the pecten oculi appears as if it would obstruct vision, it is cunningly positioned over the eye’s blind spot. The pecten is a blood vessel complex that provides extra nourishment to the eye, as well as helping protect it from damage from ultraviolet light.

Color is important in recognizing things, but it does not help nearly as much in flight or landing as edge detection. Quickly detecting contrasting edges is crucial to all animal navigation, but particularly so for fliers, and especially in low-light conditions.

Birds can see red, green, blue, and ultraviolet. Their critical edge-detection acumen is color-blind, which means it works well under any lighted condition.