Darwin’s Finches by Brian J Small (header photos from Surfbirds Galleries by Mark Gurney)

Darwin’s or Galapagos finches occur on the Galapagos archipelago, west of Ecuador, except for the Cocos Island Finch which occurs on Cocos Island, 700km NE of Galapagos.  The islands were formed through volcanic activity within the last 5 million years and have never been connected to the mainland. 

Charles Darwin during his travels on the Beagle, arrived at the Galapagos in 1835.  Whilst traveling around the islands he made some important discoveries, these inspired him to speculate on his theory of evolution via natural selection.  Some of his findings centred on the finches he found in the various habitats to be found on Galapagos.  In his Journal of Researches, commonly known as The Voyage of the Beagle, Darwin commented that, 'in the thirteen species of ground-finches, a nearly perfect gradation may be traced from a beak extraordinarily thick to one so fine that it may be compared with that of a warbler.’ 

Though we now know they inspired Darwin, we can see that they offer a clear view of adaptive radiation and allopatric speciation, where similarities in anatomy and behaviour suggests they are the product of a single colonisation followed by evolutionary divergence and speciation, through adaptation to the habitats of different islands.  Opinions differ as to the exact number of species and genera: 14 species have in the past been placed in six genera, but the recent Howard and Moore World checklist (ed. Dickinson) recognise only four genera: Geospiza (gound-finches), Camarhynchus (tree-finches, Woodpecker and Mangrove Finches), Certhidea (Warbler Finch) and Pinarolaxias (Cocos Island Finch) – Darwin was of course not aware of the presence of the last species.

There are structural and plumage differences between genera, most notably amongst males, though to some extent they are still variations on a theme. In almost all respects, the finches of the Galapagos are so similar that it is difficult to tell them apart. Indeed, it has been said that some of them look so much alike that during the mating season they find it hard to tell themselves apart.

The adaptive evolution of Darwin’s Finches is appararent in the size and shape of the beak, which is related to diet.  The Warbler Finch has a fine beak with which it feeds on small insects and spiders.  The beaks of some ground-finches Geospiza are suitable for cracking seeds, with the largest billed birds (magnirostris) being able to break larger seeds than the the smallest (fuliginosa).  Large Tree-finch Camarhynchus psittacula has a powerful beak used for extracting insects and termites from wood by bark-ripping and twig-breaking; Small Tree Finch C. parvulus has a smaller beak which it uses to glean insects from the surfaces of twigs and leaves; Vegetarian Finch C. crassirostris feeds on buds and on leaves.  The longer and decurved beaks of Common G. scandens and Large Cactus-finch G. conirostris (on Isla Genovese) are used to probe into flowers for nectar.  Two of the species in the genus Camarhynchus, Woodpecker Finch C. pallidus and Mangrove Finch C. helibates, break off twigs or leaf petioles which, by holding it in the beak, they use to to extract insect larvae and termites from beneath bark and in cavities.  Cactospiza pallidus also uses the spines from Opuntia cactus.

Darwin’s Finch facts file

• The songs of the various forms of Darwin’s Finch are very different, with no two species (as you might expect) having the same song.  Indeed there are often two or three songs within each population, rarely do males sing more than one type and populations of the same species on different islands sing different songs. Recent studies have revealed as finch populations adapt to different environments, their songs evolve in step with beak evolution, Because songs are used by females in choosing mates, the diversification of these mating signals may be expected promote speciation – the same might be said of the various forms of Crossbills that occur throughout the U.S. and northern Europe, where beak size and shape, adapted to feed on different cones, produce subtle, yet different call-notes.

• Up to ten species of finch may be seen together on the central islands of the Galapagos group. 

• In recent years only one population is known to have become extinct due to human activities on Isla Santa Cruz (G. difficilis).  After Darwin’s visit in 1835 G magnirostris became extinct on Islas San Cristobal and Floreana.

• On Isla Wolf and Darwin only, Sharp-beaked Ground-finch G. difficilis draw and drink blood from moulting boobies by pecking at the developing feathers at the bas of the tail.

• Peter and Rosemary Grant, who lived and worked on the Galapagos islands from 1973, have been the first people to study the beaks of the finches in detail. They have shown how a difference of half a millimetre in the length of a beak makes the difference between life and death for the finches during a drought.

On the Surfbirds World galleries we have been lucky to have posted nine species.  The photographs illustrate the various plumages and structure of beaks discussed above. If anybody is able to complete or fill any gaps, we would be grateful to post them and we will link them to this article.

Large Ground-finch  Geospiza magnirostris

Medium Ground-finch   G fortis 

Small Ground-finch  G fuliginosa 

Sharp-beaked Ground-finch  G difficilis difficilis,  G d debilirostris, G d septentrionalis                  

Common Cactus-finch  G. scandens scandens, G s intermedius, G s abingdoni, G s rothschildi                                              

Large Cactus-finch  G conirostris, G c propinqua, G c darwini               

Vegetarian Finch  Camarhynchus [Platyspiza] crassirostris

Large Tree Finch Camarhyunchus psittacula psitacula, C p habeli, C p affinis                      

Medium Tree Finch  Camarhynchus pauper 

Small Tree Finch  C. parvulus, C p salvini   

Woodpecker Finch C pallidus pallidus, C p productus, C p striatipectus

Mangrove Finch – C heliobates

Warbler Finch  Certhidea olivacea  -  eight subspecies – becki, mentalis, fusca, olivacea, bifasciata, luteola, cinarescens, ridgwayi

Cocos Island Finch Pinaroloxias inornata – Isla de Coco

Acknowledgements

Special Thanks to Mark Gurney for his images and comments. Ashley Banwell and Adam Scott-Kennedy for their images.