Biomechanics: September 2004

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Hummingbirds depend on a beak specially shaped for gathering nectar from flowers. But the bird must also eat insects to survive. How, then, to catch insects with a thin, nectar-drinking beak? The solution is a flexible lower bill, which widens and bends downward as the bird opens its mouth, presenting a larger “mitt” for catching flies. The action can be modeled by twisting a narrow strip of folded paper (above), as described in the text.

Roberto Osti

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.