Medium Ground-Finch (Geospiza fortis)

The highly specialised beaks of Darwin's finches enable each species to occupy a different ecological niche based on different food types (2). 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) (3). Having an intermediate sized beak, the medium ground finch is described as a generalist, able to exploit a broader range of seed sizes than the other ground finches (2) (5). Darwin's finches generally breed opportunistically with egg-laying being most profuse when rainfall is high and food abundant (3). 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 (2). 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 (2). Over the last few decades, observations of the medium ground finch population on the island of Daphne Major has provided compelling evidence of evolution in action. In particular, the beak morphology of the island's medium ground finch population has fluctuated dramatically in response to climatic conditions and competition with other ground finches (5) (6) (7).

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