Seeing eye to eye with a blue-eyed brown pelican.

A Bird's Eye View

By Bob Field

Do birds really have superior vision? Can a hawk or an eagle see details better than we can? Can owls see better than we can at night? Do any birds have panoramic vision? Yes, yes, yes, and yes.

Would you board a plane whose pilot has poor vision? If not, then you should not be surprised that our flying feathered friends have highly evolved vision. Birds also have a third eyelid, a nictitating membrane that protects their eyes. If you look closely when they blink, you can see a nearly transparent whitish membrane go across their eyes. These are very handy for animals and serve like the goggles of a swimmer or flyer.

Being eagle-eyed indicates great visual acuity- the ability to resolve details at a distance. The latest digital cameras have millions of pixels so that they can record detail almost as well as high-resolution film cameras.

Predators like hawks and eagles have ten times as many retinal cells to observe a small object, perhaps a tasty morsel. Like people, their eyes are forward-looking and the vision fields of the two eyes overlap giving them good depth perception. The only penalty they suffer is a narrower field of view than people. Birds have little if any magnifying power in their eyes. Binoculars and telescopes may be handy at times, but if you miss something outside the magnified field, it could be fatal.

Birds that are the objects of predators often have a wide field of view and their eyes are placed on the sides of their heads rather than the front. Ducks actually can see all the way around 360 degrees. While the forward-looking overlap of both of their eyes is small, they actually overlap a little in the rear as well. Like a lot of birds, ducks cock their heads sideways to look at you closely. The reason is that their best visual acuity is a small yellow spot called the fovea that contains the most sensitive cells of an eye.

Most birds turn their heads because their eyes are not mobile in the eye socket. Bird eyes are larger than the opening that you see and in fact are as large as their brains. People have one fovea per eye. Most birds - but not ducks – have two foveae per eye, one for forward-looking sight and one for side vision. The foveae are sensitive because they contain the highest density of cone shaped cells. Cones provide color vision in birds and people. Rods are very sensitive to light but not color and are important for night vision.

Owls see very well at night. Their eyes are enormous and full of rods and very few cones. They are nocturnal predators and can see in light at least three times dimmer than people can. Owls evolved to prey on small mammals, which were originally nocturnal to stay out of the way of the cold-blooded reptiles. Most mammals have very few cones if any. Most also have fur that is earth tone in color. Neither predators nor prey want to stand out.

Birds generally have excellent color vision. This should not be surprising because they evolved from reptiles that are active in daytime. Also, like many insects, they seek flowers for pollen or nectar and flowers evolved colors to standout from other vegetation.

Color vision also enables birds to use color for other purposes. Many birds are very colorful - especially the males whose displays can attract mates, drive off rivals, and draw predators away from a nest where a drab female sits motionless. In August, mallards lose their flight feathers. The males lose their colorful breeding plumage and resemble the drab but better camouflaged females.

Colors are also important for feeding the young. Gulls have red spots on their beaks that the young peck at when they want to eat. Young birds often have colorful red or yellow patterns inside their mouths to stimulate parents to feed them.

In summary, birds rely on vision in flying, hunting and gathering food, defending territory, courtship, nesting and raising young. Their behavior is a function of what they see as well as how they are seen.

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