Darwin's Famous (Un)Finches

It was one of the thrills of my natural history life to get off the aircraft at Baltra Airport, an ex-military base in the midst of a typically Galápagos lava field, and suddenly realise that the 'sparrows' around my feet were in fact the famous 'Darwin's Finches'. It was one of those literally breath-taking moments. 

Medium Ground Geospiza fortis and Large Ground Finch G. magnirostris, Baltra Airport; females or immatures.
Distinguishing them is not always easy, but that's not because they're hard to see!

I put the name in quotes advisedly; not only were they not referred to as 'Darwin's' until the 20th century, but we're very confident now that they're not finches. Their actual nature is still somewhat problematic, but the general consensus is that they're probably highly evolved tanagers, whose ancestors - probably like a grassquit, small and plain - blew across the Pacific about a million years ago.

In that time, under immense evolutionary pressures due to living in such an often brutally harsh environment, racked by regular La Niña droughts and El Niño flooding, some 13 species have developed, filling niches occupied elsewhere by groups including warblers, small woodpeckers - and of course finches. For a superbly vivid and rivetting account of their (ongoing) evolution, you can't go past Jonathon Weiner's wonderful The Beak of the Finch; evolution in real time. In large part this is the story of the remarkable Rosemary and Peter Grant, British biologists based at Princeton who have spent 6 months of each of the last 40 years (at least until recently) on little exposed Daphne Major, following, knowing and measuring every finch on the island. The rapidity of changes that they observed in species under such pressures is astounding. It's one of the great stories of modern biology.

Daphne Major at dawn. The expeditioners' landing place is among the low cliffs to the right; no beaches or jetties here!

Among the ground and cactus finches, adult males are black.

Large Ground Finch male Goepsiza magnirostris, Puerto Ayora. The bills are adapted for seed collection, different sized bills being suited to seeds of differing size and hardness.

Common Cactus Finch G. scandens, Puerto Ayora on Opuntia cactus. The bird relies heavily on the Opuntia, for nectar, pollen, fruit and seeds at different times of the year.

Large Cactus Finch Geospiza conirostris, Genovesa.
Similar to Cactus Finch but bigger and more heavily-built; restricted to just four outer islands.

 Living in such a precarious environment requires flexibility and the finches on different islands have adapted to a range of opportunities.

Small Ground Finch G. fuliginosa, Isla Fernandina, gleaning parasites and dead skin from Marine Iguanas.

In some birds the differences in beaks between species - which is the key difference for the most part - is relatively minor. It is reputed that the staff of the hugely significant Charles Darwin Research Centre say that "only God and Peter Grant" can claim to reliably assign a species to every bird encountered. 

The birds are famous because of their role in the growing understanding of the great Charles Darwin as to how species evolve, but the connection wasn't as immediate and clear-cut as is sometimes asserted - mythology has replaced history in some accounts. By that stage of the voyage, they had been away from home for nearly 4 years and were understandably anxious to get home. Perhaps partly because of this Darwin wasn't quite as meticulous as he generally was. At the time of his 1835 five week stay in the Galápagos he was still fairly conservative by the standards of his circle of peers and hadn't yet accepted the concept of species changing. It was another creationist, a young John Gould, who back home identified his finches as a group of closely related species; Darwin hadn't considered that separate but closely related species could exist on islands in sight of each other, and he had to scramble to identify exactly where his specimens had come from. (At the time he was more interested by the mockingbird species.)

It was another 10 years before he wrote, in the second edition of The Voyage of the Beagle: "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." This was truly revolutionary stuff.

For the rest, let me share with you some of the beak variations among other species.

Woodpecker Finch Camarhynchus pallidus, Los Gemelos, Santa Cruz, above and below.
In this species the sexes are indistinguishable. The strong woodpecker-like bill is used to probe crevices and rip bark in search of invertebrate prey. When times are tough in the dry season however, it famously fashions and uses cactus spines and fine sticks to extract nutritious items from hollows.

The massive parrot-like bill of the Vegetarian Finch C. crassirostris is employed to snip off buds, leaves, fruit and seeds, and to strip bark off growing shoots to access sugar-rich sap. Here is a female on Santa Cruz.

Small Tree Finch male C. parvulus, Los Gemelos, Santa Cruz.Its stubby curved bill is specialised for tweezering
insects and grubs from leaf and bark surfacesand extracting larvae from inside soft stems.

Sharp-beaked Ground Finch female Geospiza difficilis, Genovesa.
Genovesa offers the best chance of seeing this often elusive species; this is the one which, on the remote
outliers of Darwin and Wolf, has learnt to peck the base of booby feathers to obtain blood,
giving rise to the somewhat sensationalist name of 'Vampire Finch'.
While all sub-species have the sharp, more slender bill atypical of ground finches, the
others haven't adopted this behaviour.

Green Warbler Finch Certhidea olivacea, Santa Cruz.
A tiny bird with a small fine bill evolved for snapping up small insects.
Older books list jut one species, but two are now recognised, with
Grey Warbler Finch C. fusca restricted to smaller more outlying islands.

And there are another four; I've seen but not yet photographed Grey Warbler and Large Tree Finch and not yet seen the restricted range Medium Tree and Critically Endangered Mangrove Finch. Maybe one day. They are all fascinating birds and if you can possibly get to see them one day, I'd urge you to do so. You'll never be sorry.

Large Ground Finch, Puerto Ayora.

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