Jerky Birds—Part 1

Have you ever watched a bird and noticed how rapidly it jerks its head about? It will hold its head still for less than a second and then almost instantly flick its head to a new direction. It will do this so fast that I wonder how it can see at all; why things are not just a disconnected, blurry series of images. What's going on?

I've puzzled over this behavior for some time and finally did a little research (with a tip from a neighbor) to discover what seems to be the cause of this curious mannerism. What's at the base of it is the bird's constant need to be alert to predators. Small songbirds pretty much sit at the bottom of the prey hierarchy. All kinds of critters that fly and run (think hawks and cats) love to make a meal of a little bird, which must perpetually remain vigilant.

Large birds of prey—hawks and eagles—don't twitch their heads like this. In fact, they'll very slowly scan their territory, head gradually turning and eyes slowly rotating. What's more, raptors have eyes on the front of their head, to give them good depth perception, for tracking and capturing prey. Similarly, we humans require depth perception in order to catch a ball.

Vision is the dominant sense for birds. Evolution has given them very large eyes, relative to their body size. Our human eyes comprise about 2% of the weight of our heads. (My, what a large brain you have!) In contrast, the typical small bird devotes some 15% of the weight of its head to its eyes—in fact, their eyes can even outweigh their brain!

A bird has two to three times the number of cone cells than the human eye does, and we've got over four million in each eye! This gives the bird a visual acuity that we can imagine only when we use binoculars. They can see two to three times more acutely than we can. Furthermore, they can see into the ultraviolet range of the light spectrum, giving them many useful visual clues that we miss. Special eye glands emit tiny drops of oil that sharpen color perception and reduce glare. (Thus they can avoid the cumbersome burden of a baseball cap and sunglasses.)

The downside of such keen vision next time...