Medium ground-finch (Geospiza fortis)

Also known as: Medium ground finch
GenusGeospiza (1)
Weight20 g (2)


In addition to being one of Darwin’s famous finches, the medium ground-finch is celebrated as an example of evolution in action (2) (3). In appearance the medium ground-finch is very similar to the other ground finches, with the male being all black and the female being brown but streaked (3). The conventional means of distinguishing between Darwin’s finches is by the size and shape of the beak, which in the case of the medium ground finch is intermediate between those of the other ground finches (2) (3).

The medium ground-finch is endemic to the Galapagos where it occurs on the islands of Pinta, Marchena, Floreana, San Cristóbal, Santa Fe, Daphne Major, Santa Cruz, Pinzón, Rábida, Santiago, Fernandina, Isabela, Baltra and Seymour (3).

Occurs mainly in the arid lowland zone on each island (4).

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).

Like much of the Galapagos’ endemic fauna and flora, Darwin’s finches are under threat from habitat destruction, introduced diseases, and invasive predatory species such as rats and cats (8). Fortunately, the medium ground finch is still relatively abundant in parts of its range and currently is not thought to be undergoing a significant decline (9).

For their unique biological diversity and significance, the Galapagos Islands are designated both a National Park and a World Heritage Site. As a consequence, conservation of the islands’ native fauna and flora is a high priority (10). Furthermore,scientists from the Charles Darwin Foundation continue to conduct further research on Darwin’s finches in order to ensure their long-term conservation (8).

To find out more about the conservation of Darwin’s finches visit:

 For more information on this and other bird species please see:

Authenticated (21/04/09) by Professor Peter R. Grant, Department of Ecology and Evolutionary Biology, Princeton University.

  1. IUCN Red List (December, 2008)
  6. Hosken, D.J. and Balloux, F. (2002) Thirty years of evolution in Darwin’s finches. Trends in Ecology and Evolution, 17: 447 - 448.
  7. Grant, P.R. and Grant, B.R. (2002) Unpredictable evolution in a 30-year study of Darwin's finches. Science, 296: 707 - 711.
  8. Charles Darwin Foundation (March, 2009)
  9. Birdlife International (March, 2009)
  10. UNEP-WCMC (March, 2009)