"The diversity of beak structure and feeding habits within this group is remarkable. The individual species feed in a variety of ways with each specialised in a particular way. Some eat seeds, some eat insects, some remove ticks from tortoises, some eat leaves, some eat flowers, some drink blood from seabirds, and there are two species that use twigs or cactus spins to extract insect larvae from holes in the dead branches of trees. Together, they fill the roles of seven different families of South American mainland birds."
Michael Jackson, Galápagos: A Natural History
The most famous birds in the Galápagos -- aside from boobies, for entirely different reasons -- are the finches. There are 13 species of these small, usually plain birds with a lilting song, which are otherwise unremarkable except for their significance in the development of evolutionary theory. As a result, they are known collectively as "Darwin's finches" -- though he himself never thought to call them by that name.
The finches of the Galápagos are important in that all 13 species of the genus Geospiza (and one other species found on Cocos Island, 300 miles north) are quite likely descended from a single South American species, either the Blue-back grassquit or the St. Lucia black finch. In 1835, however, Darwin had no way of knowing this -- in fact he did not even recognize the nine species he collected (in the usual 19th century manner, by gunshot) as all finches, nor the finches as separate species. Soon, however, he realized that something important was going on, and began his first notebook on the "Transmutation of Species" in 1837 with special reference to the "character of South American fossils and species on Galápagos Archipelago. These facts (especially the latter) origin all my views."
What intrigues biologists about the finches of the Galápagos is the variety between populations of different islands -- particularly how the beak structure and feeding habits differ, suggesting adaptation to specific environments. Remembering that all 13 species quite possibly originated from a single pair of wind-blown grassquits fewer than a million years ago, their divergence in shape and function of bill, as well as overall morphology (body shape and size), has often been cited as the kind of "proof of evolution" creationists swear does not exist.
The evidence is all around us in the Galápagos, whether in the finches listed herein, or the island-specific variety of tortoise shell patterns, or the four species of mockingbirds, the two similar land iguanas species, or the several species of prickly pear cactus collectively known as Opuntia.
A simple case in point: in common to all 13 species is bill coloration -- adults in breeding condition have pure black bills, while those unready or unable to breed have yellow bills. Such a feature is highly suggestive of common origin for all species, and its persistence despite other changes in bill structure and function is remarkable.
The four species of ground finches are similar in coloration -- adult males are black, and females streaked brown. They all have bills of the "crushing" variety, useful for feeding on seeds. The Sharp-beaked finch (Geospiza nebulosa) of the humid highlands of the central and western islands is highly endangered, unfortunately since some ornithologists believe it may be the closest to the ancestral form of all Darwin's finches. On Wolf Island (or Culpepper, in the northwest corner of the archipelago) this finch is called the "vampire finch," for its habit of pecking at the skin of boobies until they draw blood, which they drink. Such a behavior may have evolved from eating the parasitic insects that are found in bird plumage (as ground finches do in other islands).
The Small ground finch (G. fuliginosa) is wide-spread throughout the archipelago except Genovesa and the smaller islands of the northwest, Darwin (Wenman) or Wolf; the Medium ground finch (G. fortis) is also not present on these smaller islands, Genovesa or Española (where it may be extinct). The Large ground finch (G. magnirostris), though less common than other ground finches, is found on all major islands except Genovesa and Wolf. All three species have similar appearance and behavior, and telling them apart is a challenge even for ornithologists.
Probably descendent from ground finches are the cactus finches, being more similar in coloration and distribution. The male cactus finches are mostly black, with probing bills; the females are streaked, like the ground finches. The Small cactus finch (Geospiza scandens) is found in the central islands except Fernandina, despite the presence of prickly pear cactus there (the volcanic activity of Fernandina may account for this absence). The Large cactus finch (G. conirostris) is more limited, found only on Española, Genovesa, Darwin and Wolf. Both finches are found on the large prickly pear cactus of the Galápagos, eating the small insects in the flowers or the fruit itelf.
Feeding Cactus Finch
The similar Vegetarian finch (G. crassirostris), though more closely related to tree finches, lives mainly in the humid highlands of the larger islands. It is lighter in color, more like the tree finches, though the male sports a jet-black head and neck in contrast to its creamy breast. Although its diet is primarily fruits and soft seeds, it will sneak in a few insects now and then.
Tree finches, as their name implies, are largely arboreal and feed primarily on insects. They are paler than ground or cactus finches, with gray heads and wings and white or streaked breasts. Their bill shape is sharper than ground finches, and more useful for grasping -- better suited to their insect diet. The Small tree finch (Geospiza parvula) lives on all the major islands except Española, Genovesa, Marchena and Darwin; the Large tree finch (R. psittacula) is also rare or unknown on Wolf, Santa Fé, Baltra, and Pinzón and Rábida. Of note is that it is rarely found on the large, hospitable island of Floreana.
The anomaly in this group is the Medium tree finch (G. pauper), which is as you might suspect mid-way in size between the other tree finches. However, its range is restricted only to Floreana Island, and some believe it may be a hybrid of the Large and Small tree finches found here as well. As noted, the Large tree finch is not a frequent resident of Floreana, and if the Medium species is a hybrid, it may have replaced the Large in habitat.
The miscellaneous finches are in many ways the most interesting, for here we find the most variety in behavioral adaptation. Foremost among these is the Woodpecker finch (Geospiza pallida), one of the very few birds in the world to use "tools", in this case a twig or cactus spine, to aid in feeding. The small yellowish finch will seek out and select this implement to pry insects or their larvae out of small holes in cactuses, or from beneath bark. They have even been seen to carry the tool from tree to tree, as if it has proven its value as a favorite. The woodpecker finch nests in the highlands of the major central islands including San Cristóbal, Santa Cruz, Santiago, Pinzón, Isabela and Fernandina.
Similar in appearance but much more rare is the Mangrove finch (G. heliobates), a light colored bird with narrow bill that is found on Isabela and Fernandina. Rising sea levels due to global warming post a direct threat to the survival of this species, since it nests in the endangered mangrove forests.
The Warbler finch (G. olivacea) is the smallest of Darwin's finches, and in the size and shape of its bill, coloration and song it earns its name. Even Darwin was certain it was a warbler, but John Gould -- who catalogued Darwin's collection upon the latter's return from the voyage of the Beagle -- identified it by anatomy correctly as a finch. Despite its unique role in the catalog of Darwin's finches, it is the most wide-spread species, nesting in the highlands of the large islands but being found through all quarters of the archipelago.