Raptors - Sounds Wild
Eagle Eyes


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Eagle eyes

A pair of hungry bald eagles sits in a big spruce tree overlooking a wetland. They're scanning the estuary for any sign of food - a duck or gull that might be a little impaired by an injury, or any ripples in the water revealing a fish. A small flounder in a shallow intertidal slough changes position, and one of the eagles drops from the tree in a fast glide, skims the water with outstretched talons, and scoops up the fish.

Eagles have remarkable eyes. Generally speaking, their eyesight is about eight times better than ours, in terms of visual acuity and perception. They are highly sensitive to motion and have excellent depth perception. One reason for this is they have two focal points, called fovea, on the back of each eye instead of one, like we have, for a total of four. With four fovea the eagle has the superior depth perception needed to make high speed dives and snatch up prey without missing or crashing.

An eagle's eyeball is about the same size a human eyeball, impressive considering we are basically ten to 20 times bigger than an eagle. It doesn't look as big because it is set deeper in the eagle's skull, capped by a bony ridge and surrounded by muscles. That protects the eagles' eye but limits its movement, so the eagle must turn its head to look in different directions. The eagle's eye is also protected by a second, translucent inner eyelid called a nictitating membrane that closes involuntarily when the eye needs extra coverage.