Bird beaks are more than nature's ingenious designs. They're wonders of evolution, barometers of how species react to--and sometimes even instigate--changes in their environment.
Finches with their hefty seed-crackers; warblers with their forceps made slender for extracting small insects hidden among leaves and stems; raptors with their curved hooks for tearing; shorebirds with their probes, straight or curved, which help them extract foods buried on a beach or mudflat. Novice birders quickly learn that the wild diversity of bird beaks is among the most reliable means of quickly determining to what family, and often even what species, a bird belongs. When you're faced with the bewildering array of avian life in a fall marsh or spring woodlot, that certitude is a comfort, something solid to rest on.
But it's a bit misleading, too. Birds' beaks are, in fact, always changing. They're not static over the course of an individual bird's lifetime, and they're certainly not fixed as bird species respond to--and instigate--changes in their environment. Yes, the basic order taught in Birding 101 is there. But scientists have come to learn in recent years that bills are far from being blunt instruments. Rather, they're delicate barometers of their surroundings. To examine in detail how they work is to be transported from simple satisfaction at the intricacies of efficient natural design to wonderment that evolution can get things so precisely right in so many ways.
"Birds have had this explosion of variety in their beaks," says Margaret Rubega, a biologist at the University of Connecticut. "Most vertebrate animals don't have nearly as much variation in how their jaws work. But at every stage of the process of changing beak shape it has to work; at every stage in their evolution birds had to feed themselves successfully. The bird that ends up not fed ends up dead."
Rubega began studying red-necked phalaropes in the early 1990s. Phalaropes are the black sheep of the sandpiper family: Females are larger and more colorful than males, and they take multiple mates. They feed in a distinctive manner, too, spinning like dervishes on the surface of a lake or ocean to concentrate the tiny crustaceans and other aquatic invertebrates they like to eat in a column of water, then grabbing the organisms with their long, straight bills. But the tip of a phalarope's bill is a long way from its mouth. So how does the prey make that journey? The birds don't lift their heads, which means whatever they're ingesting has to be elevated against the force of gravity. And beaks are not formed like straws, so suction can't explain what happens.
Using high-speed videography, Rubega was able to reveal what does occur. A feeding phalarope swiftly opens its bill after grabbing its prey. The food, embedded in a water droplet, races up between the bird's jaws, often in as little as one two-hundredth of a second. How does it do that? Through surface tension, Rubega found--the same force that causes water to bead on a window. Water molecules are attracted both to one another and to the molecules lining a bird's bill. As a phalarope opens and closes its bill, the molecules move closer together--which has the result of pulling the droplet up into the bird's mouth. (Recently, MIT researchers built a model phalarope bill, providing additional support for Rubega's explanation, and coined the term "capillary ratchet" for the bird's feat of biomechanical wizardry.)
Rubega observed that some birds, the star performers of the phalarope world, open their bills only a single time to raise a droplet; others need two or three tries. By looking at cross-section anatomy measurements of the birds' beaks, she was able to correlate the efficiency of a bird's feeding with its anatomy. Phalaropes have a complex series of humps on the inside of their upper jaws that increase the effect of surface tension because they give water molecules more surface area to hang onto; the more complex this topography is, the more efficiently a phalarope feeds.
"When you look at the outside of the beak, which is what scientists generally measure, there's no relationship to how well they do this transport, because that's not what makes the difference," she says. "All that matters is the internal dimensions of their beak."
One of Rubega's graduate students, Gregor Yanega, photographed hummingbirds in action to learn how these long-billed birds, so well evolved to feed on nectar, manage to capture the insect prey whose protein they also need. The question was, as Rubega puts it, "How do they manage to snatch insects with that long, delicate tool? It's as if they're using a set of chopsticks instead of a catcher's mitt."
To their surprise, Yanega's high-speed videos showed hummingbirds bending their jaws out to the sides in order to dramatically increase the size of their gape and angle the far end down to get it out of the way. In other words, they move their chopsticks out of the way in order to make better use of the catcher's mitt that lies beyond.
"We were gobsmacked," says Rubega. "We had to play the initial piece of tape back three or four times to convince ourselves that was really what we were seeing."
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.
"Conifer scales are like shingles on a roof," he says. "When crossbills are working on closed cones, they really have to exert a lot of force. Their bills are really over-powered for the seeds themselves. They're engineered to get to the seeds."
Over time, crossbill populations have come to specialize in particular seeds--such as those of black spruce or ponderosa pine--and their bills have taken on shapes that allow them to open those specific cones as readily as possible. In North America ornithologists have identified populations that specialize in at least seven different conifers; by weighing those food preferences and other characteristics, such as differing call types, they've proposed that there may be 10 species of red crossbills on this continent alone.
As crossbills have specialized, they've shaped the cones of the trees they feed on. Because getting to conifer seeds is hard work, crossbills seek out cones with thinner scales. They eat a lot of seeds, and over time their appetites have prompted conifer species to produce cones with ever-thicker scales so that some seeds are left to allow the trees to reproduce. By examining fossils, Benkman and his students have estimated how long it has taken trees to alter their cones. In Newfoundland the scales of spruce cones have grown up to 15 percent thicker in the 9,000 years since spruce, and crossbills, arrived there. On the island of Hispaniola, which has hosted crossbills for much longer, pine cone scales have become 53 percent thicker over more than half a million years.
Evidence shows this can happen even more quickly. Mauro Galetti, a conservation biologist at Universidade Estadual Paulista in Sao Paulo, has been studying how fragmented areas in Brazil's Atlantic forest respond to the removal of certain animal species. In a healthy patch of Atlantic forest, such large-billed birds as toucans and the pheasant-sized guans known locally as jacutingas feed preferentially on the largest fruits of trees, like the palm trees Euterpe edulis, which locals call palmitos. Unlike crossbills, these birds don't destroy the plants' seeds; rather, they efficiently and widely disperse them through the forest in their droppings.
When those birds vanish due to hunting and habitat destruction, there are no animals left that can disperse the larger seeds. Galetti has shown that palmitos in areas lacking large-billed birds produce substantially smaller seeds than those in pristine areas; furthermore, they're much less successful at having their seeds dispersed widely through adjacent areas. He has been able to show that this has happened in historical time--within the past two centuries. "The disappearance of animals from the forest is definitely affecting the evolution of this plant," he says.
Galetti's work is a reminder that people aren't separate from the ongoing processes of evolution. We've got a heavy hand on evolution's tiller these days. And it explains, too, why many ecologists feel such a sense of urgency at teasing out the intricacies of how the physiology of birds interacts with the environment. As Margaret Rubega notes, the feeding behavior of about three-quarters of the world's birds has never been closely studied.
"Every time we put a high-speed camera on a bird," she says, "we see something no one's seen before."