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).