Common Cactus-Finch (Geospiza scandens)

The highly adaptive beaks of Darwin's finches enable each species to occupy a different ecological niche based on different food types (2) (4). The foraging behaviour of the common cactus-finch varies with the seasons, but when Opuntia cacti are flowering, the adults use their specialised beaks to feed almost exclusively on the pollen and nectar obtained from the flowers (4) (8). At other times of the year, Opuntia fruits and seeds become important components of its diet, and during the rainy season, this finch will take advantage of various additional sources of food, such as shoots, berries, seeds, caterpillars and budworms (8) 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 to four eggs that are incubated for around 12 days before hatching. The nestlings are mostly raised on pollen, nectar and insects, and leave the nest after about two weeks (4) (9). 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 (4). Long term studies of Darwin's finches on the island of Daphne Major have provided compelling evidence of evolution in action. Over a period of several decades, the beak morphology of the island's common cactus-finch population has fluctuated in response to changes in the prevalent food source following climatic events (10) (11) (12).

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