An eagle circling three hundred feet in the air looks to us like a dot in the sky. But even from that height, the eagle is scanning the ground for prey.
To help them see small objects from a distance, birds eyes produce a telephoto effect by combining two lenses in much the same way that Galileo combined glass lenses to build some of the earliest telescopes.
Light entering our eyes passes through a tiny lens in the front of the eye and is then projected, like a slide in a slide projector, on the retina in the back of the eye.
Human Eye v. Bird Eye
In humans, the surface of the retina is smooth, like a movie screen and so the objects at the edge of our vision appear the same size as objects at the center.
In a bird's eye, a small pit on the surface of the retina acts as a second lens to enlarge a portion of the image.
Try This Experiment!
To imitate this lens, you can use a slide projector and what's called a negative lens the type used in glasses for people with myopia, or near-sightedness.
First, cut a hole the size of the lens in a piece of cardboard. Place the lens in the hole and hold it in front of the projector near the screen. You should see at the center of the screen an enlarged image of one portion of the picture from the slide.
Seeing Using A Telephoto System
Alone, the negative lens won't enlarge an image, but combined with the lens of the projector, you get the same sort of telephoto system that Galileo used in his telescopes three centuries ago and the same sort of system that birds have been using for millions of years to spot their prey from the air.