Hummingbirds, those common visitors to bird feeders and honeysuckle vines, seem adapted for one primary task: gathering nectar from flowers. Consider the apparent singularity of purpose with which these animals are shaped. Their wing bones are fused into a stiff paddle that enables them to hover at a bloom while drinking. Their tongues are long and specially shaped to hold an abundance of the sugary liquid on which they feed.
Their beaks, though, represent a truly brilliant fit between form and function. Hummingbirds with long bills feed on deep flowers, while those with shorter bills head for smaller blooms. Even the degree of bill curvature of any particular hummingbird species matches the arc of the birds’ preferred floral food source. But all is not sweet water and bliss, for the hummingbird’s beak must perform another task that seems opposed to nectar gathering—catching insects on the fly.
Nectar is the avian equivalent of Coca-Cola—it’s not much more than sugar and water. In the course of a day a hummer drinks more than its body weight in nectar, and the bird burns through the solution—which is 20 to 50 percent sugar by weight—at a furious pace. But as parents point out to their kids, you can’t live on sugar alone, and that fact is as true for hummingbirds as it is for people. The birds must supplement their carbohydrate-rich diet with daily helpings of insects to get necessary fats and amino acids that are scarce or even nonexistent in nectar.
Birds have two strategies for snagging insects on the wing. Some species with large mouths and small bills, such as nighthawks, whip-poorwills, and the aptly named frogmouth owls, open their bills wide as they fly into insects, and the prey is captured in the birds’ gaping maws. In contrast, narrow-billed birds such as flycatchers rely on their bills to catch flying prey; some flycatchers are so specialized that they maneuver the tips of their bills like forceps, pinching insects out of the air.
The hummingbird might seem best suited to the forceps approach, though a bird with such a long bill might have a hard time snapping both quickly and precisely on an evasive gnat. Gregor M. Yanega and Margaret A. Rubega, both biologists at the University of Connecticut in Storrs, have discovered that not only do hummers act more like nighthawks than like flycatchers, they also manage to widen their needlelike bill to better snag their prey.
To see how hummingbirds catch insects, Yanega and Rubega ran a video camera at 500 frames a second to film individuals of several species in slow motion as they fed on fruit flies. Projected at far slower rates, the movies reveal that a hummingbird catches flies at the base of its bill rather than at the tip. Most surprising, as the bird opens its beak to catch a fly, the lower bill suddenly bends downward at a point near the middle and widens, enlarging the bird’s mouth to the detriment of the fly.
The upper and lower parts of a bird’s beak correspond to the upper and lower jaws in people. The bones of both parts of the beak are covered with a structure known as a rhamphotheca, a sheath made of keratin, the same stuff your fingernails are made of. Unlike most vertebrates, many birds have a joint in the upper jaw, behind the beak, that enables the upper beak to bend toward the head as it opens. Some bird species also have a joint in the lower jaw, but in most species the lower bill is inflexible: it opens by rotating at the jaw joint, just like your mandible, and, also like your mandible, it does not bend in the middle. Careful examination of hummingbird skeletons shows that they belong to the latter, mainstream crowd.
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How, then, does a hummingbird manage to expand its lower bill when hunting insects? Despite decades of study, specialists were still surprised when they took a close look at this part of the hummingbird’s anatomy. It turns out that the bone of the lower beak both twists and bends like a strip of thin plastic when the lower bill opens far enough. Bones that bend? No wonder the anatomists missed that one. But bendy bones, though rare, are not unheard of. For example, the bones in a bat’s wing bend, and so do the tongue bones of woodpeckers.
You can get a feel for the remarkable bending action of the hummingbird’s lower bill by cutting a narrow strip from the long dimension of a piece of typing paper, making a rectangle one inch by eleven inches long. Fold the strip in half along the short axis, leaving a V whose two legs are each one inch by five-and-a-half inches long. Hold an end between the thumb and forefinger of each hand, with the vertex of the V pointing away from you [see illustration]. Touch your thumb knuckles together while keeping your palms parallel to the ground. The paper “bill” should now be pointed straight out in front of you, ready to open wide. Rotate your palms together and you will see that the sides of the bill spread apart while the far end rotates downward, about an axis roughly midway between your thumbs and the tip of the V. Like the narrow paper bill, the real hummingbird’s lower bill splays apart into a shape that is far better—though certainly not ideal—for catching insects.
The beak of any bird, even of a dietary specialist such as a hummingbird, serves multiple functions. Potential conflicts, such as the simultaneous need for a long, narrow bill to fit flowers and a broad bill to catch insects, often give rise to the most interesting cases of natural selection in action. The hummer resolves the conflict by devoting the shape of its bill to one purpose and its material properties to the other.