Effective feeding is not the only purpose of beaks, whose shape, size, and color affect birds’ lives in many ways. A beak’s size can dictate the notes a bird can articulate while singing; its color can attract potential mates (in many species, bright bill colors signal a healthy immune system); its shape often reflects how a species builds its nests (generally, finer-beaked birds weave more neatly). But food supply is the driving architect of shape and size.
Andrew Gosler has been reminded of that every year since 1981, when he first began measuring the sizes of great tit bills as a continuation of one of the longest-running efforts in basic natural history. Gosler, a biology professor at the University of Oxford, was interested in looking at how birds interact with habitat in Wytham Woods, a tract of land deeded to the school in 1943. Generations of Oxford researchers have been closely monitoring the plot’s woodland populations of great and blue tits—both relatives of chickadees—since 1947.
As expected, Gosler found that male great tits, which are generally larger than females, have shorter and stouter bills. He was amazed to learn that the bill of an individual great tit can vary considerably in size. That occurs, Gosler realized, because bills have to work hard, and because their underlying bony structure is covered with a layer of keratin, a tough protein related to that which forms our fingernails. “The keratin covering is continually growing, and is worn down by the food it eats and by bill-wiping behavior,” he says. “So what you see is a sort of dynamic stability between growth and wear.”
Over the years Gosler’s careful measurements revealed that great tit bills tend to be longer in summer, shorter in winter, which dovetails neatly with the birds’ needs. In summer, tits feed mainly on insects, which don’t produce much bill wear. As a result, bills grow longer and more pointed—the ideal form for probing leaf or bark crevices. In winter, tits concentrate on hard seeds, which cause a lot of wear. Bills grow shorter and stouter—and, again, this form is ideal for the work they have to do. “It’s not as if they turn from warblers into sparrows,” Gosler says, “but their beaks are changing size all the time. These little birds are superbly adapted to what they do. It’s what you would expect from an evolutionary standpoint.”
How hard birds work their bills varies a great deal. Species that feed on fruits or soft-bodied insects don’t have to deal with much abrasion, while woodpeckers or birds that concentrate on hard seeds do. But the rate of bill growth echoes the rate of abrasion—just as studies have found that the fingernails of people who bite their nails grow more quickly than those of people who don’t.
Among the champion abraders—and bill-growers—are Eurasian oystercatchers, the flashy black-and-white shorebirds that feed on rocky shores and mudflats in northern Europe. Bruno Ens of the SOVON Dutch Centre for Field Ornithology has been studying them intensively for more than three decades. “Their bills grow twice the length in a year,” he says, “so if they didn’t abrade, oystercatcher bills would be enormous.” That’s double the rate, he notes, at which fingernails grow.
Eurasian oystercatchers have three basic bill shapes so distinctive that ornithologists once suspected their bearers of belonging to different species. But that’s not the case. Rather, the form of an oystercatcher’s bill is closely shaped by the foods it eats. Some oystercatchers eat mainly marine worms; they have pointed bills that are ideal for probing in mudflats. Some hammer open cockles or mussels through brute force; they have blunt, screwdriver-shaped bills. Others feed on the same sorts of shellfish by stabbing a weak point—such as the hinge where the shell halves meet—and severing the muscle that holds the shell closed. Those birds have chisel-shaped bills.
By transferring oystercatcher chicks from parents that feed in one way to foster parents that do so in another, the researchers learned that such behavior, and bill shape, travels in families. Place the offspring of hammerers in the care of stabbers, in other words, and the youngster will learn to stab. Genetically, any oystercatcher can grow up to feed in any way—but it’s likely to do just as its parents do.
Once it does, it’s likely to keep doing what it knows, although the scientists have also proven experimentally that an adult oystercatcher can switch techniques if needed—say, when food supplies change. Its bill shape will also change while the bird goes through an awkward period of learning the new technique. As with a golfer trying to master a new swing, the transition is an inefficient, not-very-graceful process. “It takes about two weeks to change,” says Ens. “They can change, but we don’t think they do that very often. Usually they perfect their technique and get stuck on it.”
Given enough time and reliable food supplies, oystercatchers could split into three separate species specializing in different foods. That’s what appears to have happened—or is still happening—with crossbills. These birds use their eponymous bills to bite between the scales of conifer cones, then move their lower jaws to the side to spread apart the cone scales, reaching the seeds hidden underneath. This takes a lot more energy than actually cracking the seeds, says Craig Benkman, an ecologist at the University of Wyoming.