Owing to the availability of different food types, the sharp-beaked ground-finch's diet varies between the high islands and the low islands (2). Although populations on the low, dry islands mainly feed on seeds (7), they are also known to augment their diet from several unusual sources. It is on the small and remote islands of Wolf and Darwin that this species frequently drinks the blood of large seabirds, especially boobies (Sula
spp.). Alighting on the backs of the larger birds, it pecks at the feather shafts with its long, pointed beak until blood begins to flow (3) (4) (5) (6) (8). In addition, it likes to feed on the eggs of seabirds, which it cracks open against rocks (5) (8), or alternatively forage for nectar from Opuntia
catci (Wolf and Darwin) and the small flowers of Waltheria ovata
(Genovesa) (6). In contrast, while populations at high elevations also take seeds, they concentrate most of their foraging efforts in areas of deep ground litter where invertebrate prey is abundant (7). Darwin's finches generally breed opportunistically, with egg-laying being most profuse when rainfall is high and food abundant (2). Pairs are typically monogamous and maintain small territories within which they build a small dome-shaped nest in a bush or cactus. On average each clutch comprises three eggs that are incubated for around 12 days before hatching. The nestlings are mostly raised on insects and leave the nest after about two weeks (8). During the breeding season, competition for resources between different species of finch can be extremely intense. In promoting ever increasing levels of specialisation, competition for resources has been the driving force behind the evolution of Darwin's finches. This is exemplified by the widely divergent beak sizes of different finch species co-inhabiting one island, compared with much more convergent beak sizes when the same species are isolated from each other on separate islands (8).
Unique amongst birds, the sharp-beaked ground finch is famed for the extraordinary feeding habits that have earned it the sinister pseudonym of the 'vampire finch' (3) (4). Like the other ground finches (Geospiza
sp.), the adult male plumage is completely black, while the female is brown and streaked (2). However, different populations of this species exhibit greater variation in appearance and ecology than any other of Darwin's celebrated finches. Whereas some populations are similar in appearance to the common cactus-finch (Geospiza scandens
) with its long pointed beak, others are slighter in build and bare a closer resemblance to the small ground-finch (Geospiza fuliginosa
) (2) (5) (6). Indeed, so great is the morphological variation amongst populations, that questions have been raised about its status as a single species (2) (6).
The large ground-finch is endemic to the Galapagos where it occurs on the small low-lying islands of Wolf, Darwin and Genovesa, and the large, high islands of Santiago, Pinta and Fernandina (2) (6). Populations also formerly occurred at medium to high elevations on Santa Cruz, San Cristóbal, Floreana and Isabela (it is however possible that a population still exists on the volcanoes of Isabela) (6).