Darwin’s finches are distinguished by their highly specialised beaks, which enable each species to occupy a different ecological niche (6). The ground finches (Geospiza sp.) feed mainly on the ground and are generally granivorous, but also feed on arthropods and the fruit of Opuntia cacti (2) (6). In particular, small ground-finch populations in highland areas, such as on Santa Cruz, tend to spend considerably more time foraging in low vegetation (9). With its compact beak, this species is much more efficient at foraging for smaller food items than the other ground-finches, with the very small seeds of Sesuvium edmonstonei and Tiquilia fusca being typical components of its diet (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 (6).
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 (6).