By Bill Pulliam
Bend your elbow and relax your hand, preferably not the one you are using to hold up your copy of Validity. Notice what your fingers do. They settle into a loose posture where they are gently curved, and can be easily pulled, pushed, or bent. They are not completely floppy, but they are quite pliable. If you bend your knee and relax your foot, you will find your toes do more or less the same thing.
Now, imagine you are a chickadee. What would happen if you bent your leg and relaxed your little four-toed foot? Does it also rest there, half-open, with pliable toes?
Not at all. When you relax your little chickadee foot, it clamps tightly shut. When your chickadee leg bends, its leg tendons pull the foot into a compact and rigid fist. It is spring-loaded, and it would take great force to pull it open. When the leg bends, the foot clamps shut with no effort needed on the part of the bird.
To we who walk on the ground, sleep in beds, and type on keyboards, this tightly-sprung chickadee fist seems odd. But chickadees live in bushes and trees. They visit the ground only briefly. They are usually either on the wing or grasping a branch or twig. So, it is an enormous advantage to the chickadee that it requires no effort to hold on tightly to that branch. When they sleep and settle down onto the branch with bent legs, they can hold that twig all night without expending a single precious calorie.
This sort of foot with a “tendon-locking mechanism” is a universal feature among the birds known as the Passerines, commonly called the “perching birds” or “songbirds.” These include over half of the bird species on earth; the Passerines fill the second half of your bird book, from the flycatchers through the crows, titmice, warblers, sparrows, orioles, all the way to the end.
Other groups of birds also have tendon-locking mechanisms in their feet, sometimes for different reasons. Hawks have a similar setup, which keeps a tight hold on their prey items both when they first pounce on them and when they fly off carrying their catch. Woodpeckers also have locking feet, in their case to allow them to keep a firm hold on the sides of tree trunks and branches. They can even cling to the underside of a branch with very little effort. There are legends that woodpeckers have been found dead, still clinging tightly to the side of a tree. I can’t confirm that this is true, but it is not inconceivable.
Birds that swim, of course, have feet adapted to this way of life. Everyone knows that ducks have webbed toes. So do many other water birds, including cormorants, pelicans, gulls, auks and albatrosses. Loons and grebes have “lobed” feet. Each toe is shaped like a long, narrow canoe paddle, with wide flat lobes extending from its sides.
In the case of these swimming birds, it can be hard to see how these feet are actually used. Birds with fully webbed feet have two main underwater “gaits,” if you will. First is the circular paddling stroke for moving along the surface of the water. Each foot is alternately pushed back, with toes spread, then pulled forward, with toes folded. On the back stroke, the webs are fully expanded and pushing against the water; on the forward stroke they are folded like a closed umbrella to give minimal resistance.
The other underwater gait of the web-footed birds is used when they dive deep. Some ducks and other birds will plunge dozens of feet underwater, and remain down there for quite a while. To get maximum underwater speed, they will kick their feet up and down, in duck versions of the “dolphin kick” and “scissors kick” used by human swimmers.
The birds of prey use their feet as their primary hunting weapons. They carry fiercely developed claws on each toe, known as talons. The feet that bear these talons are strong and agile. Though a hawk’s primary killing and butchering tool is its beak, its feet usually make the catch and pin the quarry while it is being dispatched and eaten.
Many wetland birds that spend their time around mudflats and marshes have long unwebbed toes. These include herons, rails, and many shorebirds. The long toes spread the bird’s weight out to limit sinking, while the lack of webs reduces the accumulation of mud on the feet (think about your boots after walking through sticky mud). Some subtropical birds have toes so long that they can simply walk along the surface of lily pads without sinking. The Purple Gallinule, which you might see regularly in Florida, can accomplish this trick. But the champion water-walker is the Jaçanã, which has been nicknamed the “Jesus bird.” They are not found north of south Texas, but are common in much of Mexico, so you might see one if you vacation there. That name is Portuguese, by the way, and is pronounced something like “zhah-suh-NAW.”
Feet of course are at the ends of legs. Unlike feet, bird’s legs all have the same fundamental structure of three segments and three joints. Indeed, bird legs are not built that differently from the legs of horses, dogs and lizards. All of these have essentially the same arrangement of bones, muscles and joints. What does vary greatly between different bird species are the lengths of all these joints. Stilts, herons and cranes have proportionately long legs; swifts and hummingbirds have tiny little stumps. But these legs don’t show anywhere near the amount of structural diversity that you will find in the feet that terminate them.
Just as with its beak, you can see a bird’s lifestyle in the structure of its feet. While the bill mostly tells you what the bird eats, the feet tell you much about where and how the bird lives. Feet are often hard to see on wild birds; but this is not true of the topic of the final installment in this little trilogy of form and function. Next month we’ll talk about wings.
Bill Pulliam got started in birdwatching by his junior high science teacher in 1974, and has been an avid birder ever since in 48 U.S. states and 7 foreign countries. He is currently the Tennessee editor for eBird, an online project that compiles millions of observations from tens of thousands of birders around the world.