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.

Quirky Jerky Chickadee

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...

Summer Screamer: Cicada

Sounds from the Woods

We live in a very quiet area, so we're able to hear nature's soft sounds from quite a distance away. A thunderstorm can be heard coming from miles off. A songbird half a mile into the woods can be listened to, when there's little wind to mask it. Sometimes it's almost ghostly quiet when no animal or wind can be heard... almost like I've lost my hearing. But every now and then the silence can waft a disturbing sound my way, that is a little unnerving.

I'm sitting in the hot tub. As dusk comes on and the light level drops, the woods visually close in, while my ears can still reach deep into the gloom. I am drifting in reverie when I hear a slight rustling in the leaves, coming from somewhere back in the inky woods. I can tell it is not a little critter, like a mouse or squirrel. Maybe a deer? The leaves rustle for a moment and then go silent for a little while, as if the critter is pausing to check out its next move.

The sound is slowly moving my way. No, now I'm sure it's not a deer, since they move more quietly. It's coming closer, unnervingly noisy. I am very aware of my vulnerability—sitting naked, out there by the creek. Any determined critter could jump me before I could make it to the safety of the house.

I'm confident that a cautious animal like a deer or squirrel would stop when it spots me. But how about this critter that's approaching? Lordy, maybe it's a big ol' raccoon! Sometimes when a 'coon spots you, it'll keep advancing. All I have to defend myself is to launch a few cups of water in its direction. That might just piss him off and cause him to charge the tub.

I feel my once putty-soft body stiffen a little. I wish it wasn't so dark in those woods. Is it friend or foe? Timid or fearless? As it comes closer I begin to realize what I'm hearing is not something rustling in the leaves, but the sound of raindrops periodically falling on the carpet of leaves on the forest floor. Hmmm... how can that be? Now I remember that it sprinkled rain a little while ago. Maybe something is shaking the raindrops off the trees?

Yes, I decide it's some critter moving through the trees, dislodging drops of water and sounding like it's far larger than it probably is. I'm now thinking it's a small animal jumping from tree to tree. A squirrel, I hope? Please? In another few moments I can see a patch of nearby tree branches bouncing and then—finally to my relief—a squirrel emerges from the shadows.

Mystery solved. My heartbeat can again drop to a crawl. I contentedly watch the acrobatics of the squirrel. Beautiful!

Toad Usurping My Hot Tub

Pillbug Pervasiveness

We had a very wet spring this year—rainfall for April and May was twice normal. An abundance of rain in early spring rapidly changes the surrounding hues from winter's browns and grays to countless shades of green. It's a wonderful experience to see the growing season get such a watery boost and watch the garden vegetables luxuriate and flourish. The plants get so happy!

A wet spring causes many shifts in the balance of the local ecosystem, as compared to a dry spring. With lots of rain plants get a good season's start and insects abound, as they find many blossoms to visit. We'll be paying the price later, I fear, when mosquitoes and gnats will also proliferate. One of the unusual population explosions we've noticed this spring is pillbugs. These little terrestrial crustaceans, also known as woodlice or roly-poly bugs1, are always found in dark, wet spots. (Other delightful names are armadillo bug, doodlebug, roll-up bug, and chuggypig.) Since the wet spots have recently been everywhere, so have the pillbugs. Turn over a damp log and the bottom will be coated with these little guys. Lift some soggy mulch and bunches of pillbugs will scatter.

Pillbugs are in the family Armadillidiidae. Doesn't that make sense? A pillbug is about a quarter inch long, with 11 armored body segments. When threatened they roll into a ball, causing the critter to resemble a round little pill. This process is also known as conglobulation. (Doesn't that word just roll playfully off the tongue?) Kids call it roly-polying. These bugs also conglobulate when their environment dries up, to conserve water.

Pillbugs are one of nature's important agents for decomposing rotting wood and other decaying organic materials. They have even been kept by some people as pets—most often used to keep cages for rodents clean, by eating feces, mold, and discarded food. With that kind of diet we don't worry much about them causing any real harm, unless a bunch of them get into the strawberry beds and snack on the berries (it's their one sweet-tooth exception).

Every year is a little different in its particular mix of flora and fauna. Sometimes it's easy to ascribe and abundance (or lack) of a given plant or critter to the weather, but sometimes it also can be a mystery. If I keep a sharp eye out, maybe next year I'll solve a few mysteries.

Momma & Daughter