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

by Jonathan Weiner

  Taxonomists can be classified into splitters and lumpers. Faced with the diversity of Darwin’s finches, some splitters recognized dozens and dozens of species and subspecies. Some lumpers went so far as to call them all a single species. Generation after generation of naturalists made brief pilgrimages to the Galápagos, or puzzled over the specimens at the British Museum and the California Academy of Sciences. There were so many freaks, so many misfits that broke the serried ranks in the museum drawers. “The extraordinary variants,” one ornithologist declared in 1934, “give an impression of change and experiment going on.” Naturalists read and reread the reports of those who had seen Darwin’s finches alive, they sorted and resorted the stiff little rows of specimens in the museums, and they wondered what on earth was happening on Darwin’s islands.

  Darwin’s ground finches.

  1) Medium ground finch, Geospiza fortis. 2) Large ground finch, Geospiza magnirostris. 3) Sharp-beaked ground finch, Geospiza difficilis. 4) Small ground finch, Geospiza fuliginosa. 5) Large cactus finch, Geospiza conirostris. 6) Cactus finch, Geospiza scandens.

  Drawings by Thalia Grant

  Today most taxonomists consider the thirteen species a single family (some say subfamily) of birds. Within this family or subfamily, taxonomists think four groups of species are particularly closely related, and so, for the moment at least, most taxonomists divide the family of Galápagos finches into four genera. In one genus, the birds all live in trees and eat fruits and bugs. In the second genus, the birds also live in trees, but they are strict vegetarians. In the third genus, the birds live in trees, but they look and act like warblers. In the fourth genus, the birds spend most of their time hopping on the ground.

  This last group is the largest, with six species. It is also, for obvious reasons, the easiest to watch, and from the beginning the Grants and their team have focused on it. The Latin name of this genus is Geospiza: ground finches. The ground finches are a strange little club in themselves, a microcosm within a microcosm. The membership list includes the sharp-beaked ground finch, G. difficilis; the cactus finch, G. scandens; and also a large cactus finch, G. conirostris. Then comes a trio that has become as familiar to the Grants as Goldilocks’ Three Bears. There is a large ground finch, G. magnirostris; a medium ground finch, G. fortis; and a small ground finch, G. fuliginosa. The large ground finch has a large beak, the medium ground finch has a medium-sized beak, and the small ground finch has a small beak.

  Within each of these three species, the beaks of individual birds are variable. That is, they blur together, just as we would expect if this is a place where the river is racing and the cosmic mills are turning fast. For instance, the species in the middle of the trio, the medium ground finch, fortis, sometimes shades into the species above it, magnirostris, or the species below it, fuliginosa. The very biggest specimens of fortis are just as big as the very smallest specimens of magnirostris, and so are their beaks. At the same time the very smallest specimens of fortis are just as small as the biggest fuliginosa, and so are their beaks.

  Some of the world’s biggest fortis live on the island of Isabela; some of the world’s smallest magnirostris live on the neighboring island of Rábida. The largest of the fortis on Isabela are, even to Peter and Rosemary Grant, “almost indistinguishable” from the smallest of the magnirostris on Rábida.

  You can’t distinguish these three species by their plumage, and usually not by their build or body size either. You have to tell them apart by their beaks. In the jargon of taxonomy, the sullen art of classification, the beak of the ground finch is diagnostic: it is the birds’ chief taxonomic character. But because the finches and their beaks are so variable, many of them “are so intermediate in appearance that they cannot safely be identified—a truly remarkable state of affairs,” as the ornithologist David Lack sums up in his famous monograph, Darwin’s Finches. “In no other birds are the differences between species so ill-defined.”

  “CAUTION,” says a modern field guide to the birds of the Galápagos: “It is only a very wise man or a fool who thinks that he is able to identify all the finches which he sees.” At the Charles Darwin Research Station, on the island of Santa Cruz, the staff has a saying: “Only God and Peter Grant can recognize Darwin’s finches.”

  PETER GRANT, having studied his chaffinches, nuthatches, mice, and voles, wondered what makes some species of animals and plants hypervariable and others not. He wondered why some of the most variable species will vary even in their variability, with one flock full of eccentrics and another flock full of conformists.

  These were outstanding biological questions in the early 1970s, when Grant began looking for his next research project. (At the time, Peter was in charge of the research; Rosemary was in charge of logistics.) Theoretical and mathematical biologists were advancing paper dragons at one another in the pages of learned journals. Grant wanted to watch what actually goes on in nature. What he needed was a group of hypervariable species, well studied, variably variable, scattered across a set of remote and undisturbed locations. “The Galápagos were ideal,” he says. “Darwin’s finches were ideal.”

  The Grants made their first trip to the Galápagos in 1973. That first year they worked with one of Peter’s postdoctoral students, Ian Abbott, and his wife, Lynette, among others. To the bemusement of Peter’s and Rosemary’s families in England, the Grants also brought their two daughters, Nicola and Thalia, then aged eight and six. The girls had already camped with them in Greece, Turkey, and Yugoslavia to watch nuthatches. (In those young field days, Rosemary’s biggest job was catching Nicola and Thalia.)

  They found the birds as tame as in the days of Darwin, or of the shipwrecked bishop Berlanga, in 1535, who marveled at the birds “which did not fly from us but allowed themselves to be taken.” This is one of the strangest things about the Galápagos, after the strangeness of the animals themselves, and almost everyone who has ever written about the islands exclaims at it, including Cowley the buccaneer and Lord Byron (successor to the poet), who stopped there while returning a dead Polynesian king and queen to the Sandwich Islands.

  Cowley wrote, in 1699, “Here are also abundance of Fowls, viz., Flemingoes and Turtle Doves; the latter whereof were so tame, that they would often alight upon our Hats and Arms, so as that we could take them alive, they not fearing Man, until such time as some of our Company did fire at them, whereby they were rendered more shy.”

  Byron wrote, in 1826, “The place is like a new creation; the birds and beasts do not get out of our way; the pelicans and sea-lions look in our faces as if we had no right to intrude on their solitude; the small birds are so tame that they hop upon our feet; and all this amidst volcanoes which are burning round us on either hand.”

  “The finches are much more afraid of hawks and owls than they are of us,” Peter Grant tells friends in Princeton. “When we walk up to them, the birds keep doing what they are doing; but when an owl comes near, they head for a cactus tree. A little while ago Rosemary was crossing a treeless spot. An owl flew over, and finches flew up from all around and landed on Rosemary!

  “They are always perching on our shoulders, our arms, our heads. Sometimes when I am measuring one, a few others land up and down my wrists and arms to watch. I was looking out to sea with binoculars once, and a hawk landed on my hat. We have a picture of it.”

  “Or you pick up a bamboo pole, set it over your shoulder, and begin to carry it,” says Rosemary. “Suddenly the pole is very hard to hold up. You are walking along wondering why it is so heavy. Then you turn around and see that you have a hawk hitching a ride on the back of it.”

  “I used to have a wart on my back, although it is gone now,” says Peter. “A small black wart, up near the right shoulder. I went around in just shorts in those days, and on Genovesa, finches would peck at the wart.”

  “What a difference!” says one veteran of the finch watch, Dolph Schluter. (Schluter coined some of the team’s favorite names for itself, including El Grupo Grant, the Finch
Unit, and, for maximum grandeur, the International Finch Investigation Unit.) “In Kenya, finches flush as much as 30 meters away. In the Galápagos, the birds land on the rim of your coffee cup. If there’s only a little coffee left in there, they will land right inside it and take a sip. You can put your hand over it, and measure the bird. Mockingbirds on Genovesa would pick at our shoelaces. On the really isolated islands like Wolf, you could catch the birds by hand. Just reach out your hand and grab them.”

  Galápagos hawks. From Charles Darwin, The Zoology of the Voyage of H.M.S. Beagle.

  The Smithsonian Institution

  Peter, camping in Shiraz, had once spotted a pair of nuthatches feeding near a rock. He put a few nuts on the rock, and hid himself, hoping to observe the birds’ beaks and feeding behavior at close range. Although he waited three hours, the birds did not come back. (“It would be better to use caged birds,” Peter noted tersely in his report.) But on Daphne the most famous beaks in the world were tapping him on the shoulder. He had Darwin’s finches perching on his knee, studying him.

  Peter was keenly aware that despite the birds’ fame, despite their central place in the history of his field, no one had ever spent much time actually watching them. Darwin’s insights were strictly retrospective. David Lack’s were based mostly on inferences, museum specimens, and four months in the field. Bob Bowman had camped in the islands for less than a year.

  “I think very quickly Peter saw the Galápagos as a gold mine,” says Schluter. “Not just a wonderful place to be, but a gold mine, a treasure chest. Today, looking down the road at what one could argue is the most successful field study of evolution ever carried out, you ask yourself, Just how early in the game could Peter have foreseen all this, twenty years ago? I think maybe he had a glimmer of it from the beginning.”

  THAT FIRST YEAR the Grants and the Abbotts planned to stay in the islands for only a single season, so they worked fast, despite the heat. They studied twenty-one populations of Darwin’s finches on seven islands. At each site, at dawn, they unfurled two or three mist nets. Mist nets look rather like badminton nets on bamboo poles, but of so gossamer a weave that they are almost invisible to birds. The team left the mist nets up throughout the cool of the morning. They furled them again when the island got so hot that the birds trapped and struggling in the nets were in danger of overheating. Most days it was that hot by eight o’clock in the morning.

  The members of the team went to work on each finch they caught in much the same style they do today, armed with dividers, calipers, and a spring balance. No one had ever subjected Darwin’s finches to so many different measurements and indignities, and no one had ever measured so many finches. Over the years, in fact, the Grant team’s measurements of live Darwin’s finches have far surpassed the number of specimens in the world’s museums. Off the island of Isabela, for example, there is a group of four small islands known as Los Hermanos, The Brothers. In Los Hermanos alone, Trevor Price eventually measured twice as many living, breathing specimens of fuliginosa as repose today in museums. (The Grant team has also measured virtually every one of the thousands of museum specimens.)

  In a study of variation, everything hinges on the accuracy of the measurements. Some subjects—the dome of the shell of a turtle, the webbed feet of a duck, the diaphanous gills of a fish—are hard to measure accurately. You measure once and measure twice, and your first number is quite different from the second. If the measurements are only good give or take a few percent, and if variations from individual to individual are much smaller than that, your study is doomed from the start.

  Fortunately the finches and their beaks turned out to be not only easy to catch but also easy to measure. A long series of finch watchers could go back and measure the same bird, and they would all get numbers within a fraction of a percent of each other. Weight turned out to be unreliable, because a bird’s weight goes up and down with the time of day and the time of year. But for other measurements the difference was almost always very small. For beak length, it was only a tenth of a percent.

  “Hard facts” are those rare details in this confusing world that have been recorded so clearly and unambiguously that everyone can agree on them. The shape of a finch’s beak is a hard fact.

  The Finch Unit’s measurements not only confirmed but heightened these birds’ reputation for variability. They began to reveal how extraordinary Darwin’s finches really are. The beak of the sparrow makes a useful comparison. Sparrows are closely related to Darwin’s finches: some taxonomists place all sparrows and finches in the same family. One of Peter Grant’s field assistants that first year was the Canadian biologist Jamie Smith. Ever since the early 1970s, Smith and his own team have been conducting a parallel watch, measuring song sparrow beaks on the tiny, remote island of Mandarte, British Columbia.

  Smith has found that the beaks of song sparrows on Mandarte are all nearly the same length. It is rare to find a beak that is even 10 percent away from the mean. The probability of finding a sparrow that deviant is about four in ten thousand.

  But in the Galápagos, the Finch Unit has discovered, the probability of finding a cactus finch with a beak 10 percent from the mean is much better than four in a thousand. It is four in a hundred. One of the Grants’ world records in this respect is the depth of the upper mandible of the medium ground finch on Daphne Major. Here, the probability of finding a 10 percent deviation is one in three.

  That is one of the most variable characters ever measured in a bird. And Darwin’s finches are extraordinarily variable not only in the depth, length, and width of each mandible, and in the relative lengths of the upper and lower mandibles, but also in their wingspans, their body weights, and the lengths of their legs. Darwin’s finches are even variable in the length of the hallux, or big toe.

  Again, Darwin did not realize that these finches are so extraordinarily variable, because he did not collect enough of them to find out. Nor would Darwin have expected this result. He thought a small population would offer fewer variations from which nature can select. Hence he assumed that natural selection would be especially slow on a tiny oceanic island like Daphne Major.

  So even during their first season, the Grants and Abbotts could see that Darwin’s finches were more interesting than Darwin dreamed. And during that first field season, the island of Daphne Major sent the finch watchers an omen, a sign, a token of the difference a millimeter can make.

  One day in April of that first year in the Galápagos, after a hard day measuring finch beaks, Ian Abbott climbed down to the welcome mat on Daphne Major to take in the view. The ledge is always encrusted with barnacles, each one a crude model of Daphne itself, a cone with a hole at the top. Because Abbott was sharing the ledge with these large, sharp barnacles, he wore a pair of old shoes. But because his wife and Peter Grant were the only other human beings on the island at that moment, Abbott wore nothing besides the shoes.

  It was six o’clock in the evening, and the tide was coming in. Abbott squatted on his haunches, watching as the sun set on the neighboring island of Santa Cruz, and as hundreds of seabirds beat their way back to their roosts on Daphne Major. One millimeter beneath the future of Ian Abbott’s genetic lineage, a single barnacle towered above all the rest. And as the first waves lapped the welcome mat, this great white barnacle opened its lid, extruded its feather duster, bumped into something, and nipped shut as powerfully as only a behemoth among barnacles can.

  At least, this is how they tell it now on Daphne Major, where the story is still passed down from one generation of finch watchers to the next. They say Abbott screamed. Abbott bellowed. Abbott danced up and down on the welcome mat. At that moment he hated a barnacle as no man ever had before.

  Chapter 4

  Darwin’s Beaks

  What a trifling difference must often determine which shall survive, and which perish!

  —CHARLES DARWIN,

  letter to Asa Gray

  When Darwin was a student there, Christ’s College was know
n as the school of John Milton and the Reverend William Paley. Paley was required reading for a B.A., and Darwin read him over and over, “charmed and convinced by the long line of argumentation.” In fact, Paley’s Evidences of Christianity and Natural Theology gave Darwin “as much delight as did Euclid.”

  Natural Theology: Or Evidences of the Existence and Attributes of the Deity Collected from the Appearances of Nature had been a best seller when it was first published in 1802. The book’s first lines are sometimes quoted even today. “In crossing a heath, suppose I pitched my foot against a stone, and were asked how the stone came to be there,” Paley begins; “I might possibly answer, that, for anything I knew to the contrary, it had lain there forever: nor would it perhaps be very easy to show the absurdity of this answer. But suppose I had found a watch upon the ground.…”

  A watch would require more explanation than a stone. A watch, says Paley, implies a watchmaker. Someone had to invent it; someone had to put it together. And if that is true of a watch, Paley asks, how much more so of the living things we find on the heath? Even the simplest working parts of the smallest plants and animals go so far beyond our mortal powers of artifice that they imply “an artificer of artificers,” a creator of creators, a God.

  That was the world view of both Darwin and FitzRoy while they were standing on the black lava of the Galápagos, where a live bird looks almost as surreal as a watch on a heath. And afterward, that is how FitzRoy remembered the beaks of the Galápagos finches. “All the small birds that live on these lava-covered islands have short beaks, very thick at the blase, like that of a bull-finch,” FitzRoy writes in his Beagle memoirs (three volumes, with Darwin’s memoir tacked on as a fourth). FitzRoy is mistaken, of course; his description fits only one out of the thirteen Galápagos finches, the heavy-duty lineman’s pliers of magnirostris. But in any case, such a powerful beak—FitzRoy goes on to suggest—must be perfect for hunting and pecking on iron-hard lava and crushing berries for their juice. “This appears to be one of those admirable provisions of Infinite Wisdom by which each created thing is adapted to the place for which it was intended.”