Astute Avian Eyes

Birds' eyes are their dominant sense organ. They can hear quite well and some birds even smell well, but their sense of vision triumphs. In fact, their eyes are so large for the size of their heads that there's no room left for muscles to rotate them. They must cock their heads from side to side, in order to cover a decent field of view.

So visual acuity is definitely a bird's strong point. They can see two to three times more sharply than humans. The color and detail receptors in a bird's eye—called cones—are two to three times greater in number than for us. (That's something like 15 million cones, in a much smaller eye!) Their visual acuity leaves us far behind.

I recently watched a titmouse at the bird feeder—extracting and then rejecting at least 10-15 sunflower seeds, before becoming satisfied enough to fly off with a choice seed and commence to grasp it in its feet and bang away with its beak, to get at the sweet nut within. Most titmice (as well as their close cousins the chickadees) will fly to the feeder, quickly grab the first seed they can, and fly away to work at it. Why was this particular titmouse being so finicky—tossing away rejected seeds, until it found just the right one?

The surface of the feeder tray is strewn with hundreds of sunflower seeds and empty hulls—the latter being far dominant. When I look at the tray (to see if it's time to replenish seeds) I tend to think that there are plenty of them available, so I sometimes delay restocking the container. If I take a closer look, however (by taking the time to put my glasses on), I can usually see that most of them are empty husks. I have to look closely to determine this, yet I watch one of these birds fly to the platform, instantaneously grab a seed from all the empty hulls, and fly away to get at its interior morsel. Such keen eyesight! Their astute sense of vision is also demonstrated when I watch one of them dart to a tree trunk and pluck a tiny, juicy bug from a tiny crack.

Back to the picky titmouse who rejected a dozen or more seeds, before finally flying off with one: I'm assuming that it was looking for a fat seed, that promised a particularly large interior nut. If so, it was demonstrating an enhanced ability over its buddies, to be choosy and find that fat seed.

Why is only this one bird being so choosy? I've seen him do this trick before. While all the other titmice flit down and quickly grab a seed, this guy takes his time. Has he learned something that the others have not? Is his eyesight simply less astute? Another fascinating observation may be leading me towards one more step into the lives of these local birds.

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.

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...