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Avian Vision

April 12, 2011 | by Bruce Auchly
photo

With a 340-degree field of vision, rock doves,
or pigeons, can see almost everywhere around
them, which helps them escape predators.
(Photo by Steve Carson)

More signs of spring: meadowlarks singing on the prairie, robins chirping at dawn in town and pieces of crayfish, crawdads if you must, along the Missouri River.

Pieces of what?

As gulls return from their winter haunts they patrol the Missouri and other waters looking for crayfish, pinchers and all.

How do gulls do it? How do they and other birds find food while flying?

Whether after an insect in midair, a crayfish crawling in the shallows or a tiny berry on a tree, birds have tremendous eyesight, better than us. No surprise there.

According to the Audubon Society Encyclopedia of North American Birds, man’s vision is about equal to a bird’s, but birds have faster vision. They are quicker at picking up details.

With a single glance a bird can see as much as a human who has to scan the whole field of vision, piece by piece. And birds are better at detecting movement than us.  

Some of that advantage belongs to the size of a bird’s eyes. Depending on the bird, its eyes are the largest part of its head and often weigh more than its brain.

An adult human brain weighs about three pounds, an eye about a quarter of an ounce. Imagine us with a pair of eyes as big and heavy as our brain. That’s the stuff of bad science fiction movies.

One advantage we have is the ability to move our eyes without turning our head. Most birds cannot do that. They make up for it by having an incredible field of vision.

Pigeons, for example, have a total view of about 340 degrees. They can see almost everywhere around them except directly behind their head. That helps them avoid raptors—hawks and eagles—which have their eyes close together so they can focus on what’s in front of them, what they are trying to catch.

As a trade-off, raptors lack the side or peripheral vision that pigeons need.

When it comes to finding small objects like seeds, insects or fruit, birds rely on the cones and rods in the back of their eye.

So this doesn’t begin to sound like a visit to the eye doctor, let’s just say that one type of cells (cones) in a bird’s eyes enable it to see colors and sharp images in bright light. Another type of cells (rods) help birds like owls see at night or in low light conditions.

And another part of the bird’s retina magnifies images. It contains from twice as many to five times as many cells as in the same part of a human’s retina.

Put it all together—increased field of vision, large eye size and retina composition—and it explains how a flying gull can see a motionless piece of bread on a sidewalk or a crawling crayfish in a couple inches of water.

It must work because when was the last time you saw a bird wearing eyeglasses?