If there’s one group of Galápagos animals that’s become inextricably associated with Charles Darwin, it’s the finches. This is rather odd, because Darwin made very little mention of this group of rather ordinary looking little land birds. This raises several interesting questions: What did Darwin make of the Galápagos finches when in the islands? Why didn’t he make more of them in his subsequent writing? When, exactly, did people start to refer to them as Darwin’s finches? And why? Over the past several decades, historians have had plenty of fun finding the answers.
An Inexplicable Confusion
During his visit, Darwin found himself unable to make head or tail of the finches. ‘Amongst the species of this family there reigns (to me) an inexplicable confusion,’ he confessed in his Ornithological Notes, written around nine months after he left the Galápagos. For anyone who’s been to the Galápagos and hoped but failed to distinguish one finch species from the next, it should be of some consolation that this was Darwin’s experience too. For a start, he didn’t appreciate that they were all finches, judging the cactus finch to belong to the family that contains New World blackbirds and the warbler finch to be a kind of wren. Of those he recognised as finch-like birds, he found them rather samey, a mass of feathers ranging from light brown for the females to dark brown or black for the males, with no obvious markings to aid identification. Their behaviour also seemed rather unremarkable: ‘There is no possibility of distinguishing the species by their habits, as they are all similar, & they feed together . . . in large irregular flocks.’
But though identifying the Galápagos finches with any certainty is a feat best left to the serious ornithologist, the casual visitor will have no trouble seeing that the beaks of these birds come in an impressive range of shapes and sizes, from the very neat (as in the case of the warbler finch) to the frankly huge (like that of the large ground finch). Darwin saw this too, noting ‘a gradation in form of the bill’.
With his mind focused on geology, however, Darwin set his finch specimens aside to be described at a later date by someone who really knew his birds. That person turned out to be ornithologist and taxidermist John Gould, who judged that Darwin’s thirty-one finch specimens belonged to thirteen different species. This came as something of a surprise. Darwin, quite rightly, began to wonder how so many seemingly similar species could live alongside each other. He flipped back through his notes, wondering if he could figure out on which island he’d shot each of his specimens. He couldn’t. He got in touch with Robert FitzRoy and a couple of other Beagle hands who had assembled their own private finch collections and whose recollection of where each specimen had come from was more reliable than his own. In the end, however, the most he ever made of the Galápagos finches (at least in public) was in the beefed-up second edition of his Journal of Researches, where he drew attention to the rather impressive variation in the shapes and sizes of their beaks. ‘Seeing this gradation and diversity of structure in one small, intimately related group of birds, one might really fancy that from an original paucity of birds in this archipelago, one species had been taken and modified for different ends,’ he wrote. To illustrate his point, he included a rather nice woodcut of four finch heads, including those of the dinkily billed warbler finch at one extreme and the chunkily beaked large ground finch at the other. But as he’d only recorded the island of origin for a couple of his specimens and had not detected much difference in their behaviour, there was not much more he could say. He had little option but to leave the finches out of On the Origin of Species altogether.
FIGURE 6.1. A series of beaks belonging to Darwin’s finches. These unassuming birds have become ambassadors for evolutionary biology. This plate shows off the ‘gradation in form of the bill’ that Darwin observed, from the large ground finch (top left) to the sharp-beaked ground finch (bottom right). Reproduced from Walter Rothschild and Ernst Hartert, Novitates Zoologicae 6 (1899): 7–205.
Thankfully, another group of Galápagos birds was much easier to make sense of than the finches: the Galápagos mockingbirds. ‘I fortunately happened to observe, that the specimens which I collected in the two first islands we visited, differed from each other, and this made me pay particular attention to their collection,’ he wrote in Zoology of the Voyage of H.M.S. Beagle. So when Darwin subsequently stepped ashore on Isabela and then on Santiago, he made a special point of collecting a mockingbird from each. In total, Darwin sailed away from the Galápagos with just four birds, one from each of the islands he had visited, but he also had a chance to study other mockingbird specimens collected by FitzRoy and others. He was quick to note the similarity between the Galápagos mockingbirds and those on the South American mainland and that ‘each variety is constant in its own Island’. This observation led to Darwin’s first explicit suggestion that species might not be fixed in their nature but might in fact change. If he really was able to demonstrate that each island had just one type of mockingbird that differed from those of a neighbouring island, it would, he felt, ‘undermine the stability of Species’.
And so it did.
There are four species of mockingbird in the Galápagos, one occurring on San Cristóbal, another on Española, a third on Floreana and the most ubiquitous found on most of the other islands in the archipelago. There’s not much to separate them visually. The Floreana mockingbird is the most distinct, with a notable pale patch behind its eye and three white bands on its wing covets, features that may have piqued Darwin’s interest all those years ago. Unfortunately, the chances of seeing this particular species today are virtually nil. By the end of the nineteenth century, it had disappeared from Floreana altogether, probably as a result of depredation by introduced rats, cats and dogs and destruction of its habitat by goats. It survives still in two isolated populations on the tiny satellite islets of Champion and Gardner-by-Floreana. These are highly protected sites and off limits to tourists, but visitors to any of the other major islands are likely to be welcomed by its mockingbirds. In contrast to most other species in the Galápagos, which will pay humans little attention, these natural scavengers, which feed on everything from seeds to sea lion placenta, are always on the lookout for a stray morsel or drop of water.
FIGURE 6.2. The Floreana mockingbird. With a notable pale patch behind its eye and three white bands on its wing covets, the Floreana mockingbird clearly differs from mockingbirds on other islands. This kind of variation from one island to the next, Darwin felt, would ‘undermine the stability of Species’. Reproduced from John Gould, ‘Part 3. Birds,’ in The Zoology of the Voyage of H.M.S. Beagle, ed. Charles Darwin (London: Smith, Elder and Co., 1838–1843), 62.
So suggestive of evolution were Darwin’s Galápagos mockingbirds that in the Origin he painted a picture of how a few birds from mainland South America might have reached the archipelago, their descendants gradually populating other islands and adapting to the slightly different conditions found on each. From there he broadened out his argument: ‘We see this on every mountain, in every lake and marsh,’ he announced with a characteristic flourish.
Darwin’s Finches
In spite of the role that the Galápagos mockingbirds played in Darwin’s thinking, these mischievous little birds have been largely eclipsed by the Galápagos finches, at least in popular culture. Yet it took more than a century before biologists really picked up on Darwin’s hint and began to study this in-your-face feature—the beak—in earnest. In his 1947 book Darwin’s Finches, British ornithologist David Lack was the first to make a really good case for a relationship between beak morphology and feeding habits.
In preparation for his visit to the Galápagos in late 1938, Lack had read a couple of travel books but still found himself taken aback by ‘the inglorious panorama’. Dropping anchor just off San Cristóbal, he surveyed the small settlement that has since grown to become Puerto Baquerizo Moreno, the administrative capital of the archipelago. ‘Behind a dilapidated pier and ramshackle huts stretched miles of dreary, greyish brown thornbush, in most parts dense, but sparser where there had been a more recent lava flow, and the ground still resembled a slag heap. The land rose gradually, with no exciting features, to a sordid cultivated region, beyond which, partly concealed in cloud, were green downs, the only refreshing spot in the scene.’
In the months to come, Lack found the Galápagos more depressing still. ‘The biological peculiarities are offset by an enervating climate, monotonous scenery, dense thorn scrub, cactus spines, loose sharp lava, food deficiencies, water shortage, black rats, fleas, jiggers, ants, mosquitoes, scorpions, Ecuadorean Indians of doubtful honesty, dejected, disillusioned European settlers.’ The Galápagos finches were not much to write home about either: ‘dull to look at, not only in their orderly ranks in museum trays, but also when they hop about the ground or perch in the trees of the Galápagos, making dull unmusical noises’. He was, however, excited by ‘the variety of their beaks and the number of their species’.
The most common species, which can be seen in the arid zones of most islands, are the small, medium and large ground finches. These little birds cannot be reliably distinguished on the basis of their size or plumage, but Lack found that their classically finchy beaks suited to cracking open seeds came in three clearly different sizes. The cactus finch also feeds on seeds, though its slightly longer bill allows it to dig into the fleshy pads of Opuntia cactus for water and probe its yellow flowers for nectar. The relatively large vegetarian tree finch uses its downward curving beak to feed on leaves, buds, blossoms and fruits. Then there are the small, medium, and large tree finches, concentrated in the fertile highlands where they flit amongst the branches rather like great tits, using their tough, parrot-shaped beaks to hunt down insects. The woodpecker finch operates in a similar habitat and is similarly insectivorous, though its longer, sharper beak allows it to drill into trees for more inaccessible grubs. Although it cannot reach inside the hole with its tongue to extract a tasty morsel (as would a true woodpecker), its cunning workaround is to pick up a cactus spine or twig in its bill and use it to achieve the same end. The diminutive warbler finch’s neat, tweezer-like beak is just perfect for plucking small insects from low-lying vegetation and sometimes even from the air.
There are even more unusual feeding habits. In 1964, ornithologists following in Lack’s footsteps were the first to observe the rather gruesome behaviour of the sharp-beaked ground finch on Wolf Island. This species occurs throughout the archipelago, but on this north-westerly island, it also goes by another name: the vampire finch. It has developed a taste for blood. With their devilishly sharp bills, these small birds hop onto the back of a booby (of the red-footed or Nazca variety) and peck away at the base of the seabird’s feathers until the blood begins to flow. Then they will drink. The vampire finches will also occasionally use their bill as a lever to roll eggs out of a nest. Some even turn their beak into a kind of spring, burying it in the sand or beneath a rock and then launching backwards to give a booby egg a good kicking. Once the shell is cracked, there is something of a feeding frenzy, with several finches squabbling over the embryonic spoils.
Based on the analysis of DNA, the family of Galápagos finches is yet young, with a single species of proto-finch reaching the Galápagos somewhere between 2 and 3 million years ago. We don’t know what this species looked like for it no longer exists, but its descendants do in the shape of the fourteen species we recognise today. The lineage that led to the warbler finch was the first to appear somewhere between 1.5 and 2 million years ago, with other species appearing soon after. Of the main islands, Española has the fewest finches, with just three species. At the other extreme, both Santiago and Isabela have ten different species of finch.
This pattern is in stark contrast to that of the mockingbirds, where each island has just one species. It is just a guess, but one reason there are so many more finches than mockingbirds might be because the finches have a relatively fixed and limited song. Closely related finches notice and act on differences in repertoire, preferring to mate with individuals with a more familiar song; hence two different forms may emerge on the same island. As mockingbirds have a much more varied and flexible song and continually learn new snippets throughout life, two birds that live on the same island but sound completely different will still mate like they belong to the same species.
The Grants
Since Lack’s pioneering work, dozens of researchers have been drawn to the Galápagos finches in an attempt to make sense of this ‘inexplicable confusion’. The result is a remarkable body of work that has given us a unique insight into the process of evolution, a clear illustration of natural selection, adaptation, divergence and the origin of new species. The most significant contribution has come from two British ornithologists, Peter and Rosemary Grant, who began their study of the Galápagos finches a month before Lack’s death in March 1973. ‘In a sense, we feel we are the bearers of a torch he passed on,’ they wrote in How and Why Species Multiply in 2007.
Flying into Baltra and waiting in the queue for immigration, one can see to the west the craterous outline of Daphne Major, an island about the size of thirty football pitches where the Grants set up their operational base over forty years ago. Since then, they have been back—year in year out—typically spending six out of every twelve months furthering our understanding of these birds. ‘This is one of the most intensive and valuable animal studies ever conducted in the wild,’ wrote Jonathan Weiner in his Pulitzer Prize–winning classic The Beak of the Finch. ‘It is the best and most detailed demonstration to date of the power of Darwin’s process.’ This statement holds as true today as it did when Weiner was writing some twenty years ago.
In the first half of the 1970s, Daphne Major received plenty of rain, resulting in abundant vegetation and plenty of seeds. Concentrating on the medium ground finch, the Grants caught and banded more than 1,500 of these birds, but a drought in 1977 caused a dramatic crash in the population, with fewer than 2 in 10 birds surviving. What was really interesting, however, was that the surviving finches had bigger beaks than those that didn’t make it. The reason was a dearth of small, soft seeds of which the medium ground finches of Daphne would normally have had their fill. Only those individuals with the biggest bills were able to crack into the still abundant larger seeds. In the space of just a few months, the grim reaper’s uneven swipe through the population had caused the average depth of the medium ground finch’s beak to jump. Before the drought the average beak depth of the medium ground finch was less than 9.5 mm; afterwards it was more than 10 mm. Half a millimetre might not sound like much, but in relation to the size of these beaks, where the differences from one species to the next can be a matter of just 1 or 2 mm, it’s huge. As beak size and shape are under genetic control, the big-billed survivors passed this trait on to the next generation.
A few years later, the tables were turned with the strong El Niño of 1982–1983 that so disrupted the lives of marine creatures throughout the archipelago. The enormous amounts of rain resulted in rampant vegetation and a resurgence of small-seeded plants. Under these conditions, the medium ground finches with the pointiest beaks hoovered up, a fact reflected in the next generation of pointier billed finches.
The weather was not the only influence on the medium ground finches of Daphne Major. In the wake of the 1982–1983 El Niño, a few large ground finches reached the island for the first time. So when the next serious drought occurred in 2003–2004, the medium ground finches with the biggest beaks were unable to muscle in on the supply of larger seeds, and most of them died. The presence of two competing species effectively carried the average beak size in different directions, with that of the medium ground finch getting smaller and that of the large ground finch getting slightly larger still.
So factors like this—food abundance and competition with other species—can have a dramatic effect upon the average beak size of a population, causing it to change from year to year in subtle and sometimes exaggerated ways. For those still in doubt about what’s driving this change, it’s hard to argue with the simple fact that today’s medium ground finches on Daphne Major are smaller and have pointier bills than they did forty years ago. Then reflect that this kind of change, which even humans can detect with their relatively crude measuring tools, is by no means unique to this one species on this one island. Indeed, the average beak in every finch population of every species on every island that researchers have taken the time to look at has been in a state of flux.
So we know that the beaks of finches can change in what is effectively an evolutionary instant. But how does this give rise to new species? The most obvious mechanism is that in a relatively short period, two populations of the same species living on different islands (or maybe even on different parts of the same island) can end up looking rather different. When individuals from the two populations get together again, as they will inevitably do from time to time, they treat each other with something like suspicion, preferring to mate with birds that look and sound more like those they grew up with.
A neat experiment conducted in the early 1980s by one of the Grants’ many students simulated this kind of chance reunion. Using museum specimens stuffed in attractive poses and perched at either end of a stick, the researcher gave male finches a choice. In each trial, one of these solicitous dummies was a female from the same population as the territorial male, and the other was a very similar female from another island. It’s hard to imagine the males were not a little excited at the sudden appearance of two attractive and apparently available females, but once they’d got over their surprise, they showed a clear preference for the local female.
More important even than looks is how a finch sounds, and finches learn their relatively simple vocal repertoire from their parents early in life. By playing back sound recordings from local and more distant populations, it’s been possible to demonstrate that different finch species—even if remarkably similar to look at—can and do distinguish each other on the basis of their song. Only with this kind of discrimination can two populations become two species.
Yet such barriers to reproduction are not insurmountable, and one species of Darwin’s finch will occasionally mate with another. These unorthodox couplings, say between a medium ground finch and a cactus finch, do pretty well at first, most nests producing roughly the same number of fledglings as usual, though these hybrid youngsters struggle to make it into adulthood. On Daphne Major between 1976 and 1982, for instance, no hybrid offspring survived long enough to breed themselves. But with the arrival of the powerful 1982–1983 El Niño, which transformed the habitat, hybrids came into their own and did as well as, if not better than, either of their parental species. When this hybrid cohort came to breed, they had no trouble finding mates and produced good numbers of eggs, which hatched into healthy chicks and then fledglings.
This hybridization could play a very important role in the radiation of the Galápagos finches. When a hybrid manages to breed, backcrossing with one or other of its parental lineages, it injects some really valuable genetic combinations into the mix. This is likely to strengthen the genetic architecture of subsequent generations. It could even send a lineage in an entirely novel evolutionary direction.
From this brief survey of the Galápagos finches, it should be pretty obvious that the notion of a species is rather artificial. Darwin was well aware of the many different ways that his naturalist friends chose to define species: ‘In some, resemblance seems to go for nothing . . . in some, descent is the key—in some, sterility an unfailing test, with others it is not worth a farthing,’ he wrote to his botanist chum Joseph Hooker. ‘It all comes, I believe, from trying to define the indefinable.’ In the case of the finches, the number of species that we settle on is a balancing act, an effort to acknowledge both similarity and difference simultaneously. Being confused by Darwin’s finches is not a failing but a strength, an honest acknowledgement of the beakish continuum that befuddled Darwin himself.
Hawk and Dove
Apart from mockingbirds and finches, there are plenty of other interesting land-dwelling birds in the Galápagos. The Galápagos hawk, the only raptor and the top predator on the islands, is particularly so. Darwin found it remarkable for its vulturous habits, as reported in this rather gruesome passage: ‘When a tortoise is killed even in the midst of the woods, these birds immediately congregate in great numbers, and remain either seated on the ground, or on the branches of the stunted trees, patiently waiting to devour the intestines, and to pick the carapace clean, after the meat has been cut away,’ he wrote.
In fact, the resemblance to vultures is merely superficial. John Gould judged the Galápagos raptor to be similar to actively hunting hawks from the Americas. Recent genetic work shows that this is right and that the closest living relative of the Galápagos hawk is Swainson’s hawk. So great, in fact, is the similarity between these species that the hawks of the Galápagos are among the most recent arrivals in the archipelago, a few birds blown off their long-distance migratory path between North and South America within the last few hundred thousand years.
In that time, the descendants of these first hawkish settlers have styled themselves on vultures, with a more passive, wait-and-see approach to finding their food. They embrace a far more cosmopolitan diet than their ancestors, happily snaffling up anything from young iguanas to sea lion afterbirth. Darwin quickly realised that this behaviour might simply be explained by the principle of an animal coming to an island where it could live but finding ‘causes to induce great change’. In zoological jargon, this is known as convergent evolution, in which one creature resembles another not through common descent but because the common trait is an effective way to live.
FIGURE 6.3. The Galápagos hawk. This vulturous raptor is the top predator in the islands, with a cosmopolitan diet that ranges from young iguanas to sea lion afterbirth. Reproduced from John Gould, ‘Part 3. Birds,’ in The Zoology of the Voyage of H.M.S. Beagle, ed. Charles Darwin (London: Smith, Elder and Co., 1838–1843), 23.
Still, life in the Galápagos is clearly a struggle for this wayward hawk. This much is obvious from the number of birds that fail to breed each year, with many young or inexperienced birds hanging out in non-breeding groups. The death rate in these groups is high, much higher than amongst breeders. This is probably why more than one male and sometimes as many as eight are prepared to attach themselves to a single breeding female. This so-called polyandry is extremely rare in birds, documented in just a handful of 10,000 or so described species. The case of one female and eight males, observed on Santiago in the early 1990s, makes the Galápagos hawk one of the most extreme cases of polyandry in the avian world. Although a female hawk will lay just a couple of eggs a year, which her consorting males can have no guarantee that they’ve sired, they will still go to the trouble of feeding her and her chicks until they fledge. It’s probably worth doing so, just for the increased chance of another year’s survival and the possibility of some paternity.
Interestingly, the eradication of invasive mammals across the archipelago over the last few decades (on which more later) has had profound effects upon hawk society. This is most obvious on Santiago, where the Galápagos National Park Service shot more than 17,000 pigs and 70,000 goats between 1998 and 2006. Without a steady supply of carcasses to keep them going, the pool of non-breeding floaters evaporated. Adults too found it harder to survive, and of all breeding groups, the larger ones fared best. In years to come, this is likely to have an effect upon the Galápagos hawk’s unorthodox breeding system. With a smaller population, there may be less competition for nest sites and fewer of these male-heavy breeding groups.
If the eradication of goats has rendered the Galápagos hawk harder to see, it has led to a dramatic recovery of other birds like the secretive Galápagos rail, a small ground-dwelling species that wades its way through leaf litter in search of invertebrates like snails, beetles and ants. Back in the 1980s, for instance, a survey on Santiago detected only around twenty individuals, and it looked like the Galápagos rail might suffer the same fate as so many other island-dwelling rails. By 2005, with mammal eradication nearing completion, a similar survey located almost three hundred individuals.
Another surprising sight in the Galápagos is flamingos, a species we more commonly associate with the productive lakes of eastern Africa than an arid landscape like that of the Galápagos. But it just so happens that several brackish lagoons isolated from the ocean are home to a breeding population of the American flamingo. Their movement stirs up the mud, and they use their upturned bills to filter out bacteria, worms and crustaceans. Of all the six different species of flamingo in the world, this one is striking for the psychedelic orange of its feathers. The intensity of this colouration is down to the particular suite of carotenoid pigments contained in the microorganisms that live in the sludge they sift.
There are other wonderful avian inhabitants of the Galápagos. There are the tiny yellow warblers that flit their way through the coastal zones of most islands, nipping up hopping insects that spring from the sand. A recent study shows that as with other birds, there are clear genetic differences from one island to the next, though it looks like the first yellow warblers only reached the archipelago some 300,000 years ago and the different lineages are still in the very earliest stages of speciation. Visitors might catch sight of the Galápagos flycatcher or perhaps one of two species of owl. If they are really lucky, they may glimpse a vermilion flycatcher, the male clothed in a showy suit of red and black. Even more beautiful (in my opinion) is the Galápagos dove. It is pigeon-like in stature, yet stands out with its scarlet legs, terracotta chest and neck, gentle black and white flecks on its wings and icy-blue ring around its eyes.
The Tameness of the Birds
It’s possible to get incredibly close to the wildlife in the Galápagos. Anyone who’s been to the islands will know just how moving this is. It certainly was for the captain of HMS Blonde, George Byron. ‘The place is like a new creation,’ he wrote of his experience of the islands in 1825. ‘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 around us on either hand. Altogether it is as wild and desolate a scene as imagination can picture.’
Darwin had a similar reaction, making a special note of ‘the extreme tameness of the birds’. He wrote in his diary, ‘The birds are Strangers to Man & think him as innocent as their countrymen the huge Tortoises.’ On one day, he prodded a hawk with the muzzle of his gun; the bird fell off its branch. On another, a mockingbird alighted on the edge of a cup (tragically fashioned from the shell of a baby tortoise). Darwin reached out slowly, picked up the shell-cum-cup and lifted it—mockingbird and all—from the ground. On Floreana, he saw a boy by a well with a whip-like stick in his hand. As doves and finches came to drink, the youngster would strike them down, piling their warm bodies into a heap for his dinner. Darwin scratched his head: ‘It would appear that the birds of this archipelago, not having as yet learnt that man is a more dangerous animal than the tortoise . . . , disregard him, in the same manner as in England shy birds, such as magpies, disregard the cows and horses grazing in our fields.’
William Beebe found the Galápagos Sally Lightfoot crabs to be an exception: ‘They would always sidle out of reach, slowly if I approached gently, or like a scarlet flash if I grabbed quickly,’ he wrote in Galápagos: World’s End. Yet in one spot Beebe found three crabs, ‘which in point of fearlessness might have been the Three Musketeers’. As he waded ashore into a secluded cove on Santa Cruz, one particularly large specimen came to meet him. Beebe stood still until the crab—‘one of the biggest, his carapace fairly aflame in the sunlight’—had come within reach. Then the naturalist leaned down to rub its shell. The crab ‘sank down upon the sand, lowered his eyes into their sockets, and wiggled his maxillipeds ecstatically’. Beebe took all manner of liberties with his new friend, ‘lifting one leg after another, raising him from the ground, replacing him, standing him upon his head, and tapping gently upon his hard back’. He concluded, perhaps unfairly, that ‘this must be a very ill crab, or an idiot crustacean, or somehow abnormal.’ But much to his amazement, when he turned to leave, the tame crab followed.
The importance of this kind of experience to the modern identity of the Galápagos has not been stated forcefully enough. Getting to see a lot of interesting wildlife is one thing. Getting so incredibly close to it and being completely ignored by it is another thing altogether. It gives the concept of ‘being at one with nature’ a whole new and extremely powerful meaning, one that will sear the Galápagos experience into the mind forever. This certainly seems to have been Beebe’s reaction: ‘Once we were taught that the earth was the centre of the universe; then that man was the raison d’être of earthly evolution. Now I was thankful to realize that I was here at all, and that I had the great honour of being one with all about me, and in however small a way to have at least an understanding part.’
But it’s not, of course, just the birds that are accepting of humans. It’s everything else too, and that means the archipelago’s iconic reptiles.
