When I'm outside with a bird on my glove, I often get the feeling that I'm being watched. I turn to find Aldo the American Kestrel staring straight at me with intense focus. At least that's what it looks like. Since I'm not sure what's so interesting about me, I twist out of Aldo's direct stare. His gaze doesn't follow me; his eyes remained locked on an unknown point of interest.
In moments like this, Aldo is likely taking advantage of a unique eye adaptation. Like us, raptors have acute vision directly in front of them. We owe this detailed view of the world to our fovea, an area at the back of the eye with high concentrations of light-sensing cells. Unlike humans, raptors have a second fovea that gives an even clearer view. True to its name, this "deep fovea" has a deeper fold of tissue that magnifies images at a 45 degree angle from the eye.
Peregrine falcons are known to use this focal point while diving after prey. Instead of heading straight towards a pigeon, the peregrine curves around it in a spiral. Watching it sideways keeps the pigeon focused in the detail-oriented deep fovea. As the peregrine closes in, it will switch to the center-focus fovea to take advantage of binocular vision. With both eyes now on the pigeon, the peregrine can judge its distance with fatal accuracy.
I am always relieved to realize that when Aldo seems to stare at me like I have an embarrassing piece of food stuck in my teeth, he is actually paying more attention to something much more interesting over my shoulder.
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