by Ian Fraser
Molecular studies – including by the Grants – tell us that the Galápagos ‘finches’ are in fact tanagers (e.g. Sato et al. 2001). Tanagers, Family Thraupidae, comprise a bewildering array of mostly tropical American species. For an observer of birds, they are one of life’s delights; for a taxonomist struggling to understand their relationships, well, not so much. One such authority wrote in the highly esteemed Handbook of the Birds of the World: ‘As the taxonomic dust settles, one fact is becoming clear, namely that few avian families have been so universally misunderstood taxonomically, or have included so many taxa that have proved to be erroneously classified, as the tanagers’ (Hilty and Bonan 2017). Many are splendidly coloured, but others are not. Among the less colourfully endowed are the grassquits, which are mostly plain brown little ground-dwelling seed eaters. (For the record, ‘quit’ is generally agreed to be a 19th-century Jamaican English term for a small bird, but no-one seems able to go deeper than that. In addition to the grassquits, there’s also a Bananaquit and an Orangequit. Just saying.) The molecular studies also tell us that the closest-related living South American mainland bird to the Galápagos ‘finches’ is the somewhat dispiritingly named Dull-coloured Grassquit Tiaris obscurus from both the coast and the Andes. I have to confess that, though my records tell me I’ve seen this bird, and where, most atypically I cannot visualise the encounter! If one were anthropomorphic it could all be a bit sad actually.
Notwithstanding this, it seems that around 2.3 million years ago a flock of at least 30 birds immediately ancestral to both the Dull-coloured Grassquit and the ‘finches’ blew from the mainland and miraculously landed on the desolate exposed lava fields that are the Galápagos (Sato et al. 2001) – and they would have been especially desolate back then, relatively soon after their fiery genesis. Nonetheless, the flock managed to survive and eventually thrive. Such a mishap during a storm or cyclone is probably not uncommon, but it rarely ends well.
Some descendants, notably the ground finches, retained the deep-billed seed-eating habits of their ancestors – or rather, as seems likely, lost and then regained them. Others, the warbler finches, took up an insect-gleaning lifestyle with small sharp beaks. The cactus finches, with long sharp bills, have tied their well being to the prickly pear cactuses, consuming the flowers, fruit, fleshy pulp of the stems, and insects that feed on them (see Photo 11). All of these groups have two or three species of varying sizes to further divide up the resources. Remarkably, the Woodpecker Finch has augmented its bill by using a selected twig or cactus spine to extract larvae from tree crevices for at least half of its foraging activities; moreover, the behaviour is inherited rather than learnt. The big heavy-billed Vegetarian Finch eats a range of fruit, flowers, leaves and buds.
And all this in just 2.3 million years from apparently a single flock of small ground-feeding seed eaters. Isolation and the pressures of finding ways to compete and survive in a small world have regularly led to speciation in birds finding themselves trapped on islands, but in the case of the Galápagos they had a whole archipelago uninhabited by land birds. Perhaps some of the pioneers landed on different islands originally, or over the last couple of million years some birds have moved between islands. In the case of the outlying islands in particular, it seems unlikely that this would have been due to deliberate emigration – a small land bird really doesn’t want to fly out to sea unless it knows exactly where it’s going – so further storm-driven mishaps were probably involved, on several different occasions.
Darwin’s beach
It was an almost surreal experience to walk on Playa Espumilla (it means something like ‘foamy beach’) on the central Galápagos island of Santiago, knowing that 180 years previously Charles Darwin, accompanied by Surgeon Benjamin Bynoe and ‘some servants’, landed on this beach and walked where I was walking. Realistically, given the dynamic nature of beaches, there was more chance of me breathing a molecule of oxygen that had passed through his lungs than of my sandals walking on a grain of sand that had been in contact with his boots, but still … And some things hadn’t changed. He commented on several occasions on the tameness of the birds, and when I sat in the shade a Small Ground Finch hopped right up to me. It is an amazing place, even beyond the general amazingness of islands.
Tasmania: a continental island
The Galápagos are oceanic islands: ‘well, der’ you might say, but this term has a specific biogeographic significance. They were never part of a continent, as Tasmania, Kangaroo Island and New Guinea are integrally part of Australia, separated for now only by higher sea levels due to cyclical melting of the polar ice caps. The cycle of isolation and re-joining of Tasmania to the mainland of Australia has repeated itself roughly every 100 000 years or so for the 2.6 million year duration of the current ice age, more formally known as the Quaternary Ice Age. Temperatures drop, more and more of the Earth’s water is locked up as ice at the poles (which is by no means a ‘normal’ state of affairs in the longer history of our planet) and sea levels accordingly drop, by up to a hundred and more metres. The complex of causes, involving atmospheric and astronomic variables, the positions of the continents and the impact of those on ocean currents, is still debated by those far more erudite than I, and I’m happy to leave them to it. These glacial parts of the cycle within the overall ice age last for anywhere between 40 000 to 100 000 cold, dry and windy years, during which the Bass Strait becomes the Bassian Plain, mostly treeless but with some woodland on the western edges. Then, relatively quickly, much of the ice caps melt, sea levels rise, the world is again warmer and wetter and Tasmania is once more cut off by fretful seas.
Island endemism
In the tall eucalypts of the wet Tahune forest south of Hobart, along the Huon River where ancient Huon Pines arch over the hurrying waters, big Black Currawongs with piercing yellow eyes throw back their heads and emit a metallic ringing call like a trumpet blast. In an old garden in the central plateau, olive-brown Dusky Robins hunch on posts, waiting to drop onto unwary insects. On the north coast, at granite-strewn Bridport, Yellow Wattlebirds, their eponymous yellow wattles dangling beneath the base of their bills, shriek hoarsely while hunting cicadas in seaside eucalypts. And in the east, in Freycinet National Park, Tasmanian Scrubwrens approach through the tea-tree understorey to inspect us suspiciously as we picnic, and to chatter their disapproval. These are all Tasmanian endemics, which is to say they are not found anywhere else. In each of these cases, we are very familiar with their mainland close relations from which they are now separated – Pied Currawongs, Hooded Robins, Red Wattlebirds and White-browed Scrubwrens differ in plumage and voice, but in some cases not by a lot and the relationships are obvious.
Their ancestors became isolated from the rest of their kind when the ice melted and the Bassian Plain became Bass Strait again. Last time this happened was only 13 000 years ago however, and it’s unlikely that all these species (and eight others) evolved to be so different in such a short time. The question is, why didn’t they just meet their mainland cousins on the Bassian Plain during the many glaciations and interbreed to blend their gene pools again? I suspect that the reason lies in the conditions in Tasmania at the time, and the nature of the Plain. During the most recent glaciation, for instance, the snowline was 1000 m lower than it is today – both in Tasmania and on the mainland – and a 6000 km2 ice field, up to 70 m thick, covered Tasmania’s central highlands. Birds were not ranging across the landscape, but huddling into remnants of forest in sheltered lowland situations. The mostly treeless plain was not attractive to forest-dwelling species. Technically, birds could have crossed in both directions, and some probably did (after all there are many species which do occur on both sides of the stormy strait), but for many more species exploration was not as much a priority as was simple survival in such harsh conditions.
Tasmanian Scrubtit: an older endemic
Hobart, Tasmania’s capital, is a beautifully situated small city on the sheltered mouth of the Derwent River, snuggling up int
o the forested slopes of imposing Mount Wellington. In sheltered lower gullies, big old Soft Tree Ferns (often called Man Ferns in Tasmania) crowd for space, and a delightful network of walking tracks traverses the slopes. Above the ferns, a small rusty brown bird, like a Tasmanian Scrubwren but with pale grey cheeks, has flown up from a tree fern and is working up a eucalypt trunk, probing its curved bill into crevices for insects. This is another endemic, the Scrubtit, but, unlike the others, it has no obvious mainland counterpart and indeed is the only member of its genus. It is possible that there was a mainland member of the genus that died out, but it is more likely that the Scrubtit evolved from a mainland ancestor many glacial cycles ago and has been a Tasmanian resident for longer than the other endemics. Intuitively, we might expect that ancestor to have been a scrubwren, but a recent paper surprisingly suggests, based on molecular studies, that it probably evolved from a whiteface (Aphalocephala spp.): three species of dry country birds no longer found in Tasmania (Gardner et al. 2010).
The ‘turbo chook’: a cautionary tale
Another Tasmanian endemic bird that any visitor will surely come across is the slightly manic-looking hyperactive Tasmanian Nativehen, flocks of which will run from roadsides and ovals on sturdy legs, jinking and weaving to shelter in reed beds or in water. They have forgone the use of their wings for flight, relying instead on those strong grey legs to avoid predators, their wings swinging out to balance them as they change direction. Big birds, half a metre high, they are reputed to run at up to 50 km/h and this strategy obviously works, because they still roam in big flocks in places, despite serious persecution in the past for being ‘agricultural pests’ – they were only afforded legislative protection in 2007. Tasmanians refer to them fondly as turbo chooks: ‘chook’ being a universal Australian word for domestic hens, from an old English word ‘chuck’ or ‘chucky’, presumably from their clucking. (I sometimes fear, however, that it might be part of a uniquely Australian idiom vanishing into a homogenised television English.)
This chook has as its nearest relation the nomadic inland Australian Black-tailed Nativehen (also fondly known as ‘Barcoo bantams’): a smaller bird, not flightless but mostly found running across the ground with an erect tail like the turbo chook’s. They will appear in huge numbers in good conditions and vanish again overnight. Clearly, they form another in the now familiar pattern of species pairs created by the filling of Bass Strait (see Photo 12).
Well, no, actually. ‘Tasmanian’ Nativehens were formerly widespread on the south-east mainland until the last glaciation (i.e. from 20 000 to 12 000 years ago), when it seems that the increased aridity killed them off – though why that didn’t also eliminate them from Tasmania remains an unanswered question (Boles 2005). However, that too seems to have been a premature conclusion, because, more recently, mainland turbo chook remains from just 4700 years ago have been unearthed (BirdLife International 2016), so they were surviving until then, albeit in apparently much lower numbers, when the Dingo apparently arrived from Asia with seafaring Macassan traders. It is surely no coincidence that after this date the chooks only survived in Tasmania, where the Dingo didn’t appear. It seems that the day-active chooks could manage to survive alongside nocturnal predators, such as the Tasmanian Devil and Thylacine, but not the day-hunting Dingo.
All of which means that the nice neat pattern suggested by other Tasmanian endemic birds isn’t a template for all its endemics: did the turbo chooks evolve on the mainland (and, if so, why did they lose their flight powers there?) and later run to Tasmania, or did they evolve in Tasmania and cross to the mainland during an earlier glacial period? For now, we simply don’t have the fossil evidence to answer this question, but it remains a useful reminder to us not to be too complacent about one-size-fits-all explanations for island endemics – or, indeed, for anything else!
One possible excuse for such complacency in this situation (though we would still be wrong!) is that flightlessness is a characteristic of island bird faunas, especially among the rail Family to which the Tasmanian Nativehen belongs, so it is reasonable to assume they arose in Tasmania. Having fortuitously arrived at an island, like the grassquit ancestors of the Galápagos ‘finches’, rather than perishing at sea, there is considerable evolutionary pressure to make the most of your luck and stay there. Not that I’m suggesting any self-determination among the birds, but if I were to fall off a cliff and miraculously be saved by a protruding cliff-dwelling tree (you know how it works from watching just about any action movie), I wouldn’t be viewing this as a sign that it would be OK to jump off again. In the birds’ case, we’d expect that any genetic tendency to head out to sea again is likely to represent a dead-end line, because the chance of finding land a second time is pretty small.
Lord Howe Island: a volcanic speck
Lord Howe Island comprises the eroded remains of a volcanic crater: a speck of crescent-shaped rock, some 10 km long and 7 million years old, in the Pacific Ocean 600 km east of New South Wales. Strangely and atypically, those great seafarers, the Polynesians, seem not to have discovered the island, so that when Europeans settled there in 1834 to found a supply station for the whaling industry it was still pretty much a pristine wilderness. (Although the loss of its innocence had begun earlier than that, when Lieutenant Henry Lidgbird Ball went ashore in February 1788 to claim it for England, and begun the plunder by taking a load of turtles and ‘tame’ birds back to Sydney. Later visitors deliberately introduced pigs and goats onto the island to succour any stranded sailors.)
Island extinctions
Among the first birds encountered were two species of flightless rail, like the Tasmanian Nativehen. One of these, the Lord Howe Island Woodhen, is still with us (though it very nearly wasn’t – more on that anon). The other was a strange and beautiful big white bird with large red bill, the Lord Howe Swamphen or White Gallinule. This species scarcely survived to see the 1834 settlement; passing sailors ate it to extinction. There are just two skins of it, both in Europe, to represent the entire evolution and violent passing of a whole species – one is in Vienna, the other in Liverpool. This sad story was just the beginning of the Lord Howe bird disaster, however. The White-throated Pigeon (a subspecies of a south-west Pacific species) was abundant and was slaughtered in vast numbers. It was so tame that it was killed by sticks, and it had gone by the middle of the 19th century. The Lord Howe Parakeet made the mistake of eating the newcomers’ crops, and paid with extinction by 1869 (Hutton 1991). (There used to be some comfort in the belief that the parakeet belonged to the same species as the Norfolk Parakeet, which just clings to life on Norfolk Island, but opinion has changed on that; its passing represented the quenching of another entire species.)
Nonetheless, there was still an abundance of small bush birds, famously tame and enriching the forests, gardens and even houses of the settlement. Or they did until 1918, when a disaster even greater than the earlier ones arrived without anyone even noticing. In that year, the island trader Makambo ran aground and its cargo of Black Rats swarmed ashore. Astonishingly, within a decade, five endemic passerines, three or four of which were full species, all common and some abundant, had gone for ever. No-one will ever again enjoy or even see the Vinous-tinted Thrush (now regarded as a subspecies of Island Thrush), the Lord Howe Fantail, Lord Howe Gerygone, Robust White-eye or Tasman Starling. (There used to be a subspecies of the starling on Norfolk Island, but we dispensed with that population as well, so the species has gone for ever.) Even then we had not quite finished our fatal meddling. In a belated attempt to curb the rats (remember the Old Lady Who Swallowed a Fly?), Australian Masked Owls were introduced from Tasmania, along with some Barn Owls and, inexplicably, mainland Southern Boobooks. They didn’t get rid of the rats, but by the 1950s the endemic Lord Howe Boobook (a subspecies of Southern Boobook) had gone, probably out-competed by the two smaller invaders, and quite likely eaten by the much bigger Masked Owls.
I tell this sad story not because it is an atypical one, but becaus
e it isn’t. Islands have been witness to tragic bird stories for centuries now.
The Lord Howe Island Woodhen: almost not there and back again
Perhaps 200 bird species have become extinct since the 16th century – the exact number has to be an approximation, because some of them were not formally recorded or collected, though descriptions and even illustrations exist. This could lead to both over- and under-estimates, but any such errors probably balance out. Moreover, some apparently extinct species could, with luck, still exist in remote areas. On the other hand, some other species seem to be sliding inexorably towards extinction. At least three species (the Socorro Dove from an island off Mexico, the Guam Rail and the Alagoas Curassow from Brazil) exist only in captivity; others, including Spix’s Macaw, are teetering on that same brink. And 90–95% of these extinct species were island birds (nearly a third of them Hawaiian). One in six of all these recently extinct birds were flightless island rail species, like the Lord Howe Swamphen and Woodhen (e.g. Szabo et al. 2012).