Medium ground-finch (Geospiza fortis)

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Medium ground-finch perched on cactus, front view
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Medium ground-finch fact file

Medium ground-finch description

KingdomAnimalia
PhylumChordata
ClassAves
OrderPasseriformes
FamilyEmberizidae
GenusGeospiza (1)

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).

Also known as
Medium ground finch.
Weight
20 g (2)
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Medium ground-finch biology

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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Medium ground-finch range

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).

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Medium ground-finch habitat

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

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Medium ground-finch status

Classified as Least Concern (LC) on the IUCN Red List (1).

IUCN Red List species status – Least Concern

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Medium ground-finch threats

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).

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Medium ground-finch conservation

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).

View information on this species at the UNEP World Conservation Monitoring Centre.
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Find out more

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

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

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Authentication

Authenticated (21/04/09) by Professor Peter R. Grant, Department of Ecology and Evolutionary Biology, Princeton University.
http://www-dept-edit.princeton.edu/eeb/people/display_person.xml?netid=prgrant&display=Emeritus%2520Professors

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Glossary

Endemic
A species or taxonomic group that is only found in one particular country or geographic area.
Granivorous
Seed eating.
Monogamous
Having only one mate during a breeding season, or throughout the breeding life of a pair.
Morphology
Refers to the visible or measurable characteristics of an organism.
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References

  1. IUCN Red List (December, 2008)
    http://www.iucnredlist.org/
  2. Hau, M. and Wikelski, M. (2001) Darwin’s Finches. In: Encyclopedia of Life Sciences. John Wiley & Sons, Chichester.
  3. Grant, P.R. and Grant, B.R. (2007) How and Why Species Multiply: The Radiation of Darwin's Finches. Princeton University Press, Princeton, New Jersey.
  4. Lack, D. (1983) Darwin’s Finches. Cambridge University Press, UK.
  5. Grant, P.R. and Grant, B.R. (2006) Evolution of character displacement in Darwin’s Finches. Science, 313: 224 - 226.
  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)
    http://www.darwinfoundation.org/en/galapagos/species/birds/native-endemic
  9. Birdlife International (March, 2009)
    http://www.birdlife.org
  10. UNEP-WCMC (March, 2009)
    http://www.unep-wcmc.org/protected_areas/data/wh/galapago.html
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Image credit

Medium ground-finch perched on cactus, front view  
Medium ground-finch perched on cactus, front view

© Jurgen & Christine Sohns / www.flpa-images.co.uk

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