The Beak of the Finch: A Story of Evolution in Our Time

by Jonathan Weiner

  “People think fieldwork is so romantic,” Boag says, “but a lot of it is real slog. This was absolutely the worst.”

  They got to know the Galápagos birdseed so well that they could recognize the main species at a glance. They could often recognize a seed as it shattered in the tip of a finch’s beak. “That’s one advantage of the Galápagos,” Boag says now. “You know exactly what the birds are getting. That’s why we all want to work there. Not because it’s nice. Because it’s simple.”

  In most parts of the world, one might find two hundred species of plants in a single spadeful of earth. It would be impossible to find out exactly what a flock of birds is getting in its beaks as they move by the hour from lawns to woods to meadows to the banks of a stream. But on Daphne Major the finch watchers could get almost a God’s-eye view of their flocks, which never went anywhere and never migrated for the winter. And of course when the watchers unrolled their mist nets, adds Ratcliffe, “every bird you caught was a finch.”

  “No one anywhere has duplicated the kind of fieldwork we did in the Galápagos—because it was so simple,” Boag says. “Those ecosystems are stripped to the bare bones.”

  By the end of their first stay in the islands, the members of the Finch Unit thought they knew the finches’ tastes in seeds, fruits, insects, leaves, buds, and flowers. On Daphne Major alone the team had watched and made notes while medium ground finches ate 4,000 meals. They knew exactly what the finches were eating, and they knew the size and shape of the beaks the finches were eating them with. And most of the ground finches were eating the same seeds and fruits, just as Darwin had seen in his first glance on San Cristóbal.

  Before Peter Grant left the islands, the acting director of the Charles Darwin Research Station, Tjitte de Vries, gave him some advice. He reminded Peter that in the Galápagos the first half of each year is wet and the second half is dry. The Grant team, like Beebe, Lack, and all the rest, had visited the birds in the wet season. But the dry season might be the time to watch life squeeze Darwin’s finches.

  Darwin’s process can be hard for us to spot when nature is flush, when we behold “the contented face of a bright landscape or a tropical forest glowing with life,” as Darwin writes in Natural Selection, “… & at such periods most of the inhabitants are probably living with no great danger hanging over them & often with a superabundance of food. Nevertheless the doctrine that all nature is at war is most true. The struggle very often falls on the egg & seed, or on the seedling, larva & young; but fall it must sometime in the life of each individual, or more commonly at intervals on successive generations & then with extreme severity.”

  THE GRANTS CAME BACK a few months later. Even from the air they could see the difference as they flew in to the small airport on the island of Baltra (built by the United States Sixth Air Force during World War II and maintained now by the Ecuadoran Air Force). Everywhere the lava was brown, black, red; almost no green below the highlands of Santa Cruz. At the research station, de Vries told them that there had been no rain at all at the station in the months of April, May, June, and July.

  The Grant team recaptured many of the birds they had caught during their first trip, and dangled them again in the spring balance. The finches had lost weight, and when the members of the team counted the seeds on the same plots as before, they could see why the birds might be hungry. There was not much bird food on the ground any more. The plants had dropped all their leaves and seeds and had stopped making new ones, and the birds had eaten so many of the old seeds that their platter was almost clean. At the study site on the island of Genovesa, the volume of finch food was down by 84 percent.

  Not only was there less food for the finches, there was also less variety. Only about half of their favorite kinds of food were left. And in the wet season most of the seeds on the ground had been so small and soft that the average seed had rated only 0.5 on the Struggle Index. The seeds that remained were mostly big and tough, and the average was higher than 6.

  In the wet season every ground finch had the same favorite menu, seven kinds of soft seeds and fruits. Each ground finch had spent about half its foraging time on those seven. But now the ground finches spent only about a thirtieth of their time on them.

  Magnirostris has the biggest beak and the most powerful jaw muscles of any of the finches. It is the only finch that is strong enough to break and rip off the Grants’ metal bands from its ankles. Magnirostris was now concentrating on big, heavy seeds, the seeds that almost none of the other finches can crack.

  The long, thin beak of the cactus finch is another of the most distinctive-looking beaks among the ground finches. Cactus finches were now taking advantage of their beaks’ special talents and dining almost exclusively on cactus seeds.

  It was the same with all six of the ground finches. Now that they were reduced to tough foods, the birds’ tool-kit beaks were determining what the birds ate. They had become specialists, and each bird’s specialty was set by the shape of its beak.

  Smaller, local variations were making a difference too. The Grants can often tell which island a finch comes from just by glancing at it. Fortis on Daphne, for instance, is smaller than fortis on Santa Cruz, even though the islands are in plain sight of each other and within flying range for the finches. Meanwhile cactus finches on Daphne have narrower, finer beaks than cactus finches on Santa Cruz.

  These variations from population to population are often much subtler than the variations between species, a matter of a millimeter or so. But they too make a mortal difference. They help determine what each population can do to get through the dry season. For example, the average length of a fortis beak on the island of Pinta is slightly deeper than on Daphne Major. Torchwood stones on the two islands are almost identical in size and hardness. But on Daphne, Grant says he has seen some of the fortis take as much as six minutes to crack open a single stone. That is a long time for a bird to struggle, and most of the time the bird just gives up after a while and drops the stone. On the island of Pinta, however, the fortis—with their slightly deeper beaks—are much faster with torchwood stones, and four out of five fortis can crack them. The difference in their beaks is only a millimeter.

  Among finches of the same species on the same island, individuals often vary by smaller amounts than that. Now we are down to the level of variation that Darwin himself argued is the cornerstone of evolution. Not even Lack ever suggested that differences this small can matter in the beak of the finch. But Peter Boag made a clear and simple test of their significance a little later in the watch on Daphne Major, after hundreds of fortis had been banded. Boag walked around and around the island. Each time he spotted a fortis with a band on its ankle, he watched the bird until he saw it pick up a seed, and he wrote down what kind of seed it was. Boag found that in the dry season the birds with the biggest beaks eat the biggest seeds, the birds with medium-sized beaks eat medium-sized seeds, and the birds with the smallest beaks eat the smallest seeds: another Goldilocks-and-the-Three-Bears result.

  ONE OF THE FINCHES’ MOST BITTER STRUGGLES for existence is their running battle with a weed called caltrop. The Grants have made a case study of this. It is a classic demonstration of the war of nature, and in fact caltrop’s name is rooted in the fields of war. For more than a thousand years, soldiers have sown battlefields with a certain kind of low-tech booby trap: spiky iron balls. Generally each ball has four spikes, so that one spike always sticks up to calk, or cut, a man’s foot or a horse’s leg. Roman charioteers threw caltrops behind them to prevent pursuit. Yankee pioneers sowed the grass outside their log cabins with smaller caltrops—some called them iron stars—when there were Indians around. Caltrop, the plant, also bears the Latin name of Tribulus, from the same Latin root as tribulation: tribulare, to afflict or oppress.

  Like many other plants, including the star thistle and the water chestnut, caltrop defends its fruit with sharp spines. Each roundish fruit is divided into half a dozen sections, or mericarps, and as long as the fru
it is still on the caltrop plant, each section holds the seeds inward, to the center, with the sharp spines facing out. When the fruit dries, these mericarps fall one by one to the ground. There is a single row of seeds nestled inside each mericarp, like peas in a pod. One mericarp holds as many as half a dozen large, nutritious, nutty-flavored kernels, each kernel in its own woody compartment, like chocolates wrapped in wooden foils inside a locked wooden box.

  A caltrop’s mericarp can be awkward in a finch’s beak, almost as awkward as an iron star beneath a human foot or a horse’s hoof. In fact, two species of finches on Daphne Major, the cactus finch and the small ground finch, have never been seen to try to open them. The only species that do attack mericarps are the large and the medium ground finches, magnirostris and fortis, and each species has its own tactics.

  Caltrop. The large armored object in the center is a caltrop fruit. When it dries, it breaks into pieces called mericarps. The mericarps hold three to six seeds apiece. In this picture, finches have taken one seed from the mericarp at the bottom, and all of the seeds from the mericarps to its left and right, leaving small, black hollow cells where the seeds had been.

  Drawing by Thalia Grant

  Magnirostris (whose beak is almost twice as wide and twice as deep as the beak of a fortis) picks up a mericarp, holds it near the midpoint of its beak, and squeezes its mandibles together. After a while the mericarp shatters into fragments. Then magnirostris picks up each fragment, holds it on one side of the beak, and crushes it. “When a mag is working on a Tribulus,” Peter Grant says, “I can hear it cracking.”

  To crack a whole mericarp like this takes an average force of more than 200 newtons. Apparently that is more force than a fortis can muster. Instead it braces the mericarp against the ground and bites and twists the woody sheet that guards the row of seeds, as if peeling off a lid. This operation requires about 54 newtons of force, which seems to be about the best a fortis can do.

  Neither species has it easy, and Grant has seen birds of both species use a rock to help them. The finch holds the mericarp in its beak, braces the upper mandible against a rock, and squeezes the lower mandible closed on the seed while pressing the upper mandible against the rock.

  Now, magnirostris, because it crushes the whole mericarp in its powerful beak, can eat every last seed before it moves on to another mericarp. But fortis, with its smaller beak and weaker jaw, has to peel off the lid, exposing and eating one seed at a time. Typically it eats only one or two seeds and then moves on. It almost always eats the seeds in the same order too, starting at the narrow, pointed end of the mericarp and working toward the blunt end, as charmingly methodical as a child eating corn on the cob.

  Watching these birds combing the dry lava for Tribulus seeds is like watching people hunt through a bowl of pistachio shells for the last unopened nuts, the ones that were thrown down before as too tough to crack. Birds of both species will often pick up a mericarp, work at it for a few seconds—sometimes longer—and then drop it and move on, like someone dropping a sealed pistachio nut back in the bowl. The finches prefer mericarps with only two spines, and mericarps with four spines are likely to be dropped. One indication that magnirostris has an easier time than fortis at eating caltrop, says Grant, is that a magnirostris cracks many more mericarps than it rejects, while a fortis rejects many more than it cracks.

  Medium ground finch. From Charles Darwin, The Zoology of the Voyage of H.M.S. Beagle.

  The Smithsonian Institution

  Darwinian competition is not only the clash of stag horns, the gore on the jaws of lions, nature red in tooth and claw. Competition can also be a silent race, side by side, for the last food on a desert island, where the competitors never fight one another, and the only sound of battle is the occasional crack of a Tribulus seed. Finches are locked in the most deadly competition even when they feed together in flocks. When times are hard, their lives depend on how efficiently they can forage for food—how little energy they can expend in getting how much energy in return. They are hungry, they are thirsty, and they are trying to keep their budget in balance. And as poor Mr. Micawber used to say, “Annual income twenty pounds, annual expenditure nineteen nineteen six, result happiness. Annual income twenty pounds, annual expenditure twenty pounds ought and six, result misery.”

  The race is to the swift, and magnirostris is clearly the winner. It can eat more than four seeds from two mericarps in less than a minute, while fortis gets only three seeds from two mericarps in more than a minute and a half. In fact magnirostris gets about two and a half times as much energy per minute, and because it gets more seeds out of each mericarp it has to hop around less too, which also saves energy.

  Of course, magnirostris is bigger than fortis, not only in beak but in body size, so it needs more food. It needs one and a half times as much energy to meet its minimum daily metabolic requirements. But since the big beak of magnirostris earns the bird two and a half times as much energy, it still comes out ahead.

  A few fortis have found a trick that helps them even the score. One of them sometimes trails a magnirostris around on the lava. As soon as the magnirostris cracks a mericarp, Grant says, the fortis rushes up, steals a piece, flies a little way off, and cracks it. Not every fortis on Daphne seems to know this trick; the Grant team has spotted only about half a dozen of them doing it. (Likewise on Daphne Major, cactus finches sometimes open cactus buds, and fortis has never been seen even trying to open them. But sometimes a fortis will wait beside a cactus finch, scandens, and after the scandens has torn open the bud, the fortis joins in.)

  So the trials and tribulations of caltrop are not only harder on fortis than magnirostris; they are harder on some fortis than others. Fortis with bigger beaks can crack the mericarp and gouge out the seeds faster than those with smaller beaks. Tiny variations are everything. A fortis with a beak 11 millimeters long can crack caltrop; a fortis with a beak only 10.5 millimeters long will not even try.

  “The smallest grain in the balance” can decide who shall live and who shall die. Between a beak big enough to crack caltrop and a beak that can’t, the difference is only half a millimeter.

  Incidentally, the beak of the finch may be exerting selection pressure on the caltrop itself. The Grants have not made a careful study of this. But out of curiosity Peter once compared the caltrop on the eastern rim of the crater, where there is heavy fortis traffic, and the caltrop on the northwest inner wall of the crater, about 20 meters down from the rim, where fortis rarely goes. Where there are many finches, each mericarp has fewer seeds, but it has longer and more numerous spines. In the steep, rugged, protected place, the mericarps have more seeds and fewer, shorter spines.

  Peter suspects that the caltrop is evolving in response to the finches. Where the struggle for existence is fierce, the caltrop that is likeliest to succeed is the plant that puts more energy into spines and less into seeds; but in the safer, more secluded spot, the fittest plants are the ones that put more energy into making seeds and less energy into protecting them. The finches may be driving the evolution of caltrop while caltrop is driving the evolution of the finches.

  According to the greatest authority on plants of the Galápagos, Duncan Porter, this species of caltrop comes from Africa. It may have traveled across the Pacific from island to island on the boots, pantaloons, and hairy legs of sailors, whalers, and buccaneers. If that is how it reached the Galápagos, then the very earliest date for its arrival on Daphne Major is 1535, which is the year the first European saw the Galápagos (it was the unlucky Fray Tomás de Berlanga, the third bishop of Panama, who was so glad to get away from the islands that afterward he did not even bother to name them).

  We will never know when the first Tribulus seed reached Daphne Major. Probably it was not the year the good bishop collided with the islands, for he is unlikely to have seen Daphne Major, or to have stopped there if he did. So the war of beaks and spines on this islet may very well have evolved in the brief space of a few centuries—since the f
irst seabird landed on the crater floor with a caltrop stuck to its webbed foot, or since the first human being sailed around the island a few times and planted a boot on the welcome mat.

  “WHAT A TRIFLING DIFFERENCE must often determine which shall survive, and which perish!” Darwin wrote. To many of his critics this has seemed pure conjecture, but after a good part of a lifetime on Daphne Major, the Grants find it obvious. “I often think of piano playing,” Rosemary Grant says. “I know I try to play the piano in spite of small hands, and how much easier it would be if my fingers were only a little longer. Or think of tweezers,” she adds. Everyone in El Grupo Grant needs tweezers, because they collect cactus spines, especially during a nest census or a seed-and-fruit census, and some of Daphne Major’s longest-suffering human inhabitants have come to consider tweezers “the most indispensable item of Daphne equipment.” A complete tweezers kit includes a slant tip, a square tip, and a pointed tip. Often you can use any one of them to do the job of another, but that makes clumsy work and takes a long time. “And what a small difference in shape and size there is between the different kinds!” says Rosemary.

  The Grants see the importance of variations on display not only on Daphne Major but throughout the archipelago. The second most closely watched island in their study is Genovesa, which the Grants began watching intensively in 1978. The work on Daphne is mostly Peters, and the work on Genovesa is mostly Rosemary’s. When they are alone the Grants call Daphne “Peter’s Island” and Genovesa “Rosemary’s Island.”

  On Genovesa, Rosemary has focused on the cactus finch conirostris (a finch that Darwin never saw). These cactus finches, like their congeners on Daphne, all eat more or less the same food when the food is cheap, but in times of famine they tend to specialize. Those with significantly longer beaks can hammer open the fruits of the cactus and probe the cactus flowers. Those with longer and deeper beaks can crack the big, tough cactus seeds. Those that have significantly deeper beaks than the others can strip the bark from the trees to get at the bugs beneath. It is one more demonstration that Darwin was right about the importance of slight variations. All this is precisely the kind of illustration that Darwin asks his readers to imagine in the Origin.