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

by Jonathan Weiner

  In some way Cocos finches are more diverse than Galápagos finches. Some eat bugs, others crustaceans, still others nectar, fruits, seeds. There are specialists that eat mostly on the ground, others in bushes, others in tall trees. Normally to assemble a list of birds with such a wide range of specialties one would have to collect not only many species, but many genera, or even many families, groups of genera. Yet the finches of Cocos Island are a single species.

  These finches cannot diverge and ramify on Cocos the way their siblings have in the Galápagos archipelago. On Cocos Island they can’t get away from one another. The place is too small, and the nearest land is too far away (Cocos is about 500 kilometers, more than 300 miles, from the coast of Costa Rica). There is no chance for geographic isolation. Insects and snails can radiate in such a place, because for them even a tiny island is big enough to present, in effect, an archipelago of isolated habitats, but birds do not radiate on single isolated islands.

  That, of course, is why our own kind is not radiating into new species. For us the whole planet is almost as small now as Cocos is for the Cocos finch.

  Rosemary and Peter Grant have not studied the Cocos finches. The most careful work on them to date has been conducted by another married couple of evolutionists, Tracey Werner and Tom Sherry. In the 1980s they spent a total of four seasons watching the finches, concentrating on a single big thicket of hibiscus. There they banded, watched, and measured about one hundred finches, in the style of the Grants (in fact, Nicola Grant was one of their field assistants). They watched their hundred finches eat, season after season, recording and analyzing a grand total of 26,770 feeding attempts.

  The bodies of the finches on Cocos Island are far more uniform in size and shape than in the Galápagos, and they all carry the same pointed, slender, all-purpose beak. Their specialties have nothing to do with their sex or age, or with minute variations in the size and shape of their beaks. Nor is a Cocos finch’s food choice tied to the seasons, the time of day, or some particular spot in the hibiscus thicket. Werner and Sherry saw the finches working away at their chosen trades indefatigably at all times and places; they often saw half a dozen finches each going after food in his or her own way at the same time in the same bush, like blacksmiths, bakers, printers, and tailors in the shops around a village green. Some gleaned bugs from branches, some probed the branches, some gleaned from living leaves, some from dead, curled leaf clusters, and some sipped nectar from flowers. Whatever their métier, they kept at it hour after hour, day after day.

  Most of these trades required skilled labor. For instance, Werner says, Cocos finches that probed branches for bugs had to pry, strip, and twist bark off the branch to get at the insects below, as tree finches do in the Galápagos. She says many of the finches’ trades were not only skilled but highly specialized: some finches in the hibiscus thicket spent most of their time picking out tiny moth larvae from between leaf layers in morning glories.

  Werner and Sherry suspect that the finches were learning all these specialized trades in the way that human beings do: from their elders. The finch watchers often saw a young bird hopping along a short way after a grown one, watching and copying. The juvenile would observe, hop to exactly the place the adult had just left, and do what the adult had just done at that spot.

  Werner and Sherry also saw the juveniles hopping along after warblers and sandpipers, watching and copying them. The young fledglings also flocked together, in groups of anywhere from two to thirty, like teenagers at a shopping mall, and they watched and copied one another too.

  Evolutionists know that lions, apes, elephants, and other social mammals learn from their elders, but they tend to think of birds as inheriting their skills simply through instinct. In the staggeringly rich rain forests, however, as the evolutionist Jared Diamond has observed, there may be an adaptive advantage for birds that can learn from their elders. “An insectivorous bird is faced with tens or hundreds of thousands of local insect species,” Diamond notes, “including more than a thousand beetle species in one tree species alone.” Even trained human beings—rain-forest shamans and entomologists with Ph.D.s in rain-forest ecology—have to spend decades, entire lives, learning to recognize a small fraction of this diversity. Perhaps finches in the Cocos rain forest have been selected for their learning ability, just as human beings were selected for it at the dawn of our species.

  Compared with other birds, the Cocos finches struck the finch watchers as awkward, even clumsy, feeders. They often saw the birds lose their grip while looking for bugs, and fall out of the trees. Sherry once watched a juvenile trying to peel bark from a branch. The finch fell, hovered, perched, peeled, fell, hovered, perched, peeled—over and over, until he finally extracted a prize, a one-inch-long centipede. Another time, Sherry saw a finch trying to catch a spider on a branch. “The bird lunged for the spider and missed,” Werner writes in her thesis on the Cocos finches. “The spider dropped down, hung from its thread. The bird scrambled down the trunk of the tree.” Each time it came alongside the spider the finch took another stab and missed again. The spider kept spinning out thread, and the bird kept hopping alongside it trying to keep up: leaping, lunging, missing, all the way down the tree to the ground. There the spider made a mad dash—and another finch got it.

  A finch watcher once saw a Cocos finch give the same bumbling kind of chase on a liana. “It seemed odd,” he wrote afterward. If the bird had used its wings it probably could have outmaneuvered the spider very quickly, he thought. Yet it chose instead to sideslip slowly and awkwardly down the vine.

  This sounds like the sort of comedy that other animals might enjoy every day if they watched us doing what we do. Compared with fish we are bad at swimming, compared with birds we are stiff at flying, compared with cheetahs we are ludicrous at running, compared with ants we are hellacious at cooperating. Yet we are the most successful species of our time. We have overrun and overturned the territories of all these other animals because taken as a whole, by learning from the generation before us, we can do a fair job at all of their skills at once. As the evolutionist Ernst Mayr has written, we have “specialized in despecialization.”

  Our position on the planet is the same as the Cocos finches in their private rain forest. The range of foraging opportunities that lies open to us vastly exceeds our ability as individuals to take advantage of them all. Like the finches we have evolved an extraordinary ability to learn, so that as a species, collectively, we can take advantage of these myriad niches, and we keep finding more and more trades. We fill more different ecological niches than any other animal.

  This is what allows us to carry on the epic learning game we call science. Science formalizes our special kind of collective memory, or species memory, in which each generation builds on what has been learned by those that came before, following in each other’s footsteps, standing on each other’s shoulders. Each generation values what it can learn from the one before, and prizes the discoveries it will pass on to the next, so that we see farther and farther, climbing an infinite mountain.

  “I FIND IN [the] animal kingdom,” Darwin scribbled to his botanist friend Hooker, “that the proposition that any part or organ developed normally (ie not monstrosity) in a species in any high or unusual degree, compared with the same part or organ in allied species, tends to be highly variable. I cannot doubt this, from my mass of collected facts.—To give instance, the Cross-Bill is very abnormal in structure of Bill compared with other allied Fringillidae, & the Beak is eminently variable.”

  Hooker wrote Darwin that he did not see this phenomenon in plants. Darwin shot back, “I daresay the absence of Bot. facts may in part be accounted for by the difficulty of measuring slight variations. Indeed after writing this occurred to me; for I have Crucianella stylosa coming into flower & the pistil ought to be very variable in length, & thinking of this I at once felt how could one judge whether it was variable in any high degree. How different, for instance, from Beak of Bird!—”

 
Darwin develops this theme in the Origin in a chapter titled “Laws of Variation,” under the heading, “A Part Developed in Any Species in an Extraordinary Degree or Manner, in Comparison with the Same Part in Allied Species, Tends to Be Highly Variable.” Here Darwin cites the beaks of domestic pigeons, when the beak is part of what distinguishes the breed.

  This pattern holds across nature, and of course the beaks of Darwin’s finches have turned out to be a particularly dramatic case. From the moment they hatch, these species differ more from one another in their beaks than in any other character the Grants have measured. No other character in Darwin’s finches is as variable as the beak.

  In our own species, it is the human brain that has diverged most dramatically from those of our nearest relatives on the tree of life. True to Darwin’s law the human brain is exceedingly variable in its measurements, like the beaks of Darwin’s finches, pigeons, and crossbills. The volume of the human skull, its cranial capacity, is more variable than the depth of fortis beaks on Daphne Major.

  The mind is our beak, and the human mind is even more variable than the brain. Darwin, for instance, was extraordinarily gifted at observing, collecting, theorizing, and also at finding mentors. But he was a poor mathematician. He wrote to a friend at school who had not answered his letters, “I suppose you are two fathoms deep in mathematics, and if you are, then God help you, for so am I, only with this difference, I stick fast in the mud at the bottom and there I shall remain.” Which turned out to be true. (That is one reason Darwin’s theories are more accessible than Newton’s. He expressed even his deepest thoughts in a language that anyone can read.)

  Not everyone in a village or neighborhood can master every trade, and no one man or woman can master more than a few, but if people specialize, then collectively the villagers have a hundred trades. Their infinite variety of minds and talents helps them to radiate into all these crafts and specialties.

  “I believe that this psychological polymorphism has been a major reason for the success of the human species,” says J. B. S. Haldane. Variation is the secret of our adaptive radiation. There is infinite value for the species in our infinite variety. As among Darwin’s finches, it is the tool by which we get more of the food available to us than we ever could if each of us tried to do it all, if each human being tried to be a generalist. Our minds and talents are variable for the same evolutionary reason as finches’ beaks are variable in the Galápagos: jack-of-all-trades, master of none. And what drives this radiation within our species is a process like character divergence. Though we may not think of it as Darwinian, we all feel its pressure, wanting and needing to do what we are made for—seeking the task for which we are most fit.

  Emerson writes, “Each man has his own vocation. The talent is the call. There is one direction in which all space is open to him. He has faculties silently inviting him thither to endless exertion.” William Blake: “How ridiculous it would be to see the Sheep Endeavoring to walk like the Dog, or the Ox striving to trot like the Horse; just as Ridiculous it is to see One Man Striving to Imitate Another. Man varies from Man more than Animal from Animal of different Species.” Aeschylus: “Character is destiny.”

  Those whose lives are comfortable believe themselves more or less insulated from the pressures of natural selection, because they have the leisure and the freedom to make so many choices in their lives. But they are ruled by selection as much as any other creatures on the planet, for they too are using their own individual variations to lessen the pressure of natural selection. Everywhere very young human beings start out acting more or less alike, just as fledglings on Daphne start out using their diverse beaks in more or less the same experimental ways. As we get a little older we enter a phase of wild experimentation, as finches also do on Daphne. As we get older yet we narrow our efforts, again like the finches. In every country, within the limits of our choices and opportunities, we tend to seek trades in which we have learned by experience that we are unlikely to lose, to be killed or driven out by the competition, trades in which our weaknesses will do us minimal harm. We try to find the work for which our beak best fits us—although what we find in the end is rarely perfect, as Darwin’s contemporary, the witty parson Sydney Smith, observes,

  If you choose to represent the various parts in life by holes upon a table, of different shapes—some circular, some triangular, some square, some oblong—and the persons acting these parts by bits of wood of similar shapes, we shall generally find that the triangular person has got into the square hole, the oblong into the triangular, and a square person has squeezed himself into the round hole. The officer and the office, the doer and the thing done, seldom fit so exactly that we can say they were almost made for each other.

  From which the phrase “A square peg in a round hole.” And as Mark Twain adds, “A round man cannot be expected to fit in a square hole right away. He must have time to modify his shape.”

  IN THIS WAY we have become the evolving animal. We are now evolving rapidly ourselves, and we are driving evolution everywhere around us. We have learned how to make Darwin’s process run faster for us than it does for any other species on the planet—except perhaps the bacteria, with their flying rings of plasmids and ten-minute generation times. The tragedy of our success is what we are doing to the rest of creation, which evolves more slowly.

  Our own tenure has been brief, and on average the term of a species is brief—a few million years. A species that can survive only by causing upheaval around it is in constant danger of extinction, like a tribe that lives for battle. At the moment the whole planet is like a closed pinecone that we alone with our twisted beaks have contrived to open, so that there are more of our kind now than of any other bird in the forest. Yet the rapid accumulation of change is not always progress, and forward motion is not always an advance.

  When they visit cactus flowers, cactus finches on Daphne Major sometimes snip the stigma, which is the top of the hollow tube that pokes out like a tall straight straw from the center of each blossom. When the stigma is cut, the flower is sterilized. The male sex cells in the pollen cannot reach the female sex cells in the flower. The cactus flower withers without bearing fruit.

  These finches are, of course, completely dependent on the cactus. Without cactus pollen, cactus nectar, cactus fruits, and cactus seeds, they would starve. The birds’ fates are so closely bound up in the fates of the cactus that when there is more cactus on Daphne Major, there are more cactus finches on Daphne Major; when there is less cactus, there are fewer finches.

  Stigma snipping.

  Drawing by Thalia Grant

  On Daphne one December near the start of the cactus-flowering season, the Grants peeked into more than two thousand cactus flowers. Almost half the flowers had lost their stigmas. On Genovesa the next month the Grants examined more than a hundred cactus blossoms, and four out of five of them had been violated. In some years the cactus finches have destroyed almost every cactus flower, and in those years the cactus on their island produced virtually no fruits or seeds. It is hard to imagine a simpler, neater, faster way for a species of Darwin’s finches to drive itself toward extinction.

  To find out what was going on, the Grants took turns, rotating every two hours all day long, keeping watch over seventeen cactus flowers. They noted when each flower opened, which finch fed at each flower, and what each finch did. Cactus flowers usually open in the morning between 9 and 11. When a cactus finch lands beside an open flower, it holds the stigma to one side with its foot so that it can nibble on the pollen that is cupped in the base of the flower. But sometimes a cactus finch will visit a cactus bud very early in the morning, an hour or two before opening time, and pull apart the folded petals to get in there before anybody else. When the flower is pried half-open, the stigma is liable to poke the finch in the eye. On Genovesa, in the year of their stigma study, the average stigma stuck out of the cactus flower about 25 millimeters; the average distance from the tip of a cactus finch beak to its eyeball was onl
y 21 millimeters. So, when it forced open the flower bud, the finch would sometimes snip the stigma with its beak and flick it away. Not all of the cactus finches on the island were doing this, the Grants discovered: only about a dozen.

  A finch that snips stigmas is like a farmer who eats seed corn. The bird steals from its future and the future of its line. By sterilizing flowers, the cactus finches on Daphne Major cut each year’s harvest in half. And all the stigma snippers get out of it is a little pollen the other birds can’t get, and a bit of nectar the other birds can’t reach—a sweet treat in the early morning.

  Darwin’s process cannot stop this dirty dozen, and in fact the process favors them, because the stigma snippers pay no special price for their stolen sweets. They are no more likely to die in the dry season than the birds that spare the stigmas. In dry seasons, cactus finches forage on one another’s cactus anyway; they don’t stay within the borders of their own territories when they are hungry. So the dozen flower violators on the island are trampling on a commons, not on their own private gardens. A finch that spares the flowers on its territory cannot guarantee itself a good meal later in the year when times are hard. In fact, as the Grants point out, a bird that takes good care of all of the stigmas on its property may even encourage trespassers later.

  A cactus finch. From Charles Darwin, The Zoology of the Voyage of H.M.S. Beagle.

  The Smithsonian Institution

  Natural selection turns upon the profit of the individual. What is good for the individual is usually good for the flock. But when the needs of the individual clash with the needs of the flock, it is the individual that triumphs, even if this private success leads to the downfall of the flock. If the terrible drought of 1977 had been followed by a second year just as dry, all of the cactus finches on Daphne would have been in jeopardy because of the stigmas that the dozen birds had severed. Those birds might have made the difference between survival and extinction on Daphne Major.