Jerky Birds—Part 2

These ocular avian advantages come at a cost, however. With such massive eyes crammed into the tiny skull of small birds, there is no room for muscles that rotate the eyes, so they have to turn their head to change their field of view. In addition, in order to have a wide field of view—remember all those predators—their eyes are located on the sides of their head, so they can see almost all the way around themselves. The cost for this capability is they have no depth perception, because each eye views a different image.

It's pretty hard to tell how far away things are if you have no depth perception. That can be fatal, if a hawk is rapidly zeroing in on your location. (There are no one-eyed baseball players, since catching a ball would be pure luck with no depth perception.) So what's a vulnerable little bird to do? He moves his head rapidly, from one position to another. This action accomplishes two things: one, he can triangulate on objects and two, he can better detect a moving object. Triangulation can help discern the form of and distance to things. When the head stabilizes for an instant, the bird can better detect motion “out there.”

Songbirds, being tiny little critters, move much faster than we humans can. Their eyes can resolve rapid movements—about twice as rapidly as we can. So they flick their heads about, helping them quickly to spot and track the movements of any potential predator. It's their critical ability to reconnoiter their immediate territory, without it all becoming a blur.

I got my first clue about what's going on with the jerky habits of birds when a neighbor described to me how he spots deer when he's hunting. The best indicator of a deer's presence to him is detecting its motion, since Mother Nature has gifted deer with such excellent camouflage. If a human is continually meandering through the woods, he's likely to miss the slight movements of a deer (who is always on the alert for predator motion). My neighbor demonstrated his hunting technique to me: quietly, yet fairly quickly taking three steps and then freezing like a statue for a couple of seconds, while his eyes sweep the area. That gives him a much better chance of spotting a deer.

After that demonstration, the more I watched birds twitch their heads about, the more I saw that it resembled my neighbor's hunting technique. With that hint to guide me, matched to the amazing ability of Internet search engines, I was on my way.