The small ground finch (Geospiza fuliginosa) is a species of bird in the tanager family Thraupidae. Endemic to the Galápagos Islands, it is common and widespread in shrubland, woodland, and other habitats on most islands in the archipelago. It commonly feeds on small seeds and parasites from the skins of Galápagos tortoises, and Galápagos land and marine iguanas.
Taxonomy and systematics
The small ground finch is one of Darwin's finches, a group of closely related birds which evolved on the Galápagos Islands. The group is related to the Tiaris grassquits, which are found in South America and the Caribbean.
When Charles Darwin first collected the species in 1835, he thought it was a finch. John Gould, who officially described Darwin's specimens, agreed, placing it in the genus Fringilla with the Old World finches. By 1841, Gould had changed his mind, moving this and five other species into the new genus Geospiza — still a genus of finches, but distinct from those of the Old World. DNA research has now shown that all Darwin's "finches" are actually tanagers.
The name Geospiza is a combination of the Greek words geo-, meaning "ground-", and spiza, meaning finch. The specific name fuliginosa is late Latin for "sooty".
It is known to hybridize (rarely) with the medium ground finch.
This is the smallest of the ground finches, measuring 11 cm (4.3 in) in length .[nb 1] Its beak is short and pointed, with a slightly curved culmen. On average, its beak is smaller than that of the medium ground finch, though there is a significant overlap in size between the two, particularly on islands where only one of the two species exists. On islands where the two species compete directly, the differences between their beaks are greater. The male is black with white-tipped undertail coverts, while the female and young are brown with streaked underparts. There have been observable phenotypic differences between finches that live in lowlands and ones that live in highlands, and this change is most likely attributed to adaptation.  The finches seen in highlands have larger, more pointed beaks and smaller feet and claws compared to the lowland variety. These finches are on a cline (series of biocommunities on a continuous gradient), and individuals in the hybrid zone have intermediate traits. This is an example of parapatric speciation, where the elevation gradient of 560 meters is causing differentiation in traits, but hybrids are well adapted in their “hybrid zone.” 
Habitat and range
Like all but one of the other Darwin's finches, the small ground finch is endemic to the Galápagos Islands. Abundant and widespread, it is found on every island in the archipelago except Genovesa, Wolf and Darwin. It is most common in arid coastal and transition areas, though it moves into the highlands following the breeding season.
Like the other Galápagos ground finches, the small ground finch is an omnivore with a preference for vegetable matter. It feeds primarily on the ground or in low vegetation, eating seeds, buds, flowers, leaves and the occasional insect. It forms symbiotic relationships with Galápagos tortoises and both marine and Galápagos land iguanas, gleaning parasites from their skins.
The small ground finch's song is rapid and weak, transcribed as "twichooo-twichooo" or "teur-weee".
Conservation and threats
Although the population size of the small ground finch has not been quantified, it is described as common across the Galápagos, and the International Union for the Conservation of Nature lists it as a species of Least Concern. Its numbers seem to be stable, and neither its population size nor its range size appear to approach thresholds for concern. However, like all endemic wildlife on the Galápagos Islands, it is impacted by some human activities, including fires, overgrazing by domestic and feral animals, and the introduction of exotic species. It is found in ten of the Important Bird Areas established on the islands. The species suffers from high mortality rates from the parasitic fly, ranging from 16% to 95% over a four year period (2002-2006). 
- ^ By convention, length is measured from the tip of the bill to the tip of the tail on a dead bird (or skin) laid on its back.
- ^ a b BirdLife International (2012). "Geospiza fuliginosa". IUCN Red List of Threatened Species. Version 2012.1. International Union for Conservation of Nature. Retrieved 16 July 2012.
- ^ Newton, Ian (2003). Speciation and Biogeography of Birds. San Diego, CA, USA: Academic Press. p. 78. ISBN 978-0-12-517375-9.
- ^ Donahue, Kathleen (2011). Darwin's Finches: Readings in the Evolution of a Scientific Paradigm. Chicago, IL, USA: University of Chicago Press. p. 154. ISBN 978-0-226-15771-9.
- ^ Tudge, Colin (2008). The Bird: A Natural History of Who Birds Are, Where They Came From, and How They Live. New York, NY, USA: Random House. p. 183. ISBN 978-0-307-34205-8.
- ^ Jobling (2010), p. 172
- ^ Jobling (2010), p. 165.
- ^ Grant (2008), p. xvi.
- ^ a b c d e f Swash, Andy; Still, Rob (2005). Birds, Mammals, and Reptiles of the Galapagos Islands: An Identification Guide (2 ed.). London, UK: Christopher Helm. p. 102. ISBN 978-0-300-11532-1.
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- ^ Rice, Stanley A. (2007). Encyclopedia of Evolution. New York, NY, USA: Facts on File. p. 114. ISBN 978-1-4381-1005-9.
- ^ http://osu.worldcat.org/title/adaptive-divergence-in-darwins-small-ground-finch-igeospiza-fuliginosai-divergent-selection-along-a-cline/oclc/5164006122&referer=brief_results
- ^ Sulloway, F, & Kleindorfer, S (2013). Adaptive Divergence in Darwin's Small Ground Finch (Geospiza fuliginosa): Divergent Selection along a Cline. Biological Journal of the Linnean Society, 110, 45-59.
- ^ Eibl-Eibesfeldt, Irenäus (1996). Love and Hate: The Natural History of Behavior Patterns. Hawthorne, NY, USA: Aldine de Gruyter. p. 38. ISBN 978-0-202-02038-9.
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- ^ Stattersfield, Alison J. (1998). Endemic Bird Areas of the World: Priorities for Biodiversity Conservation. Cambridge, UK: BirdLife International. p. 171. ISBN 978-0-946888-33-7.
- ^ "Small Ground-finch (Geospiza fuliginosa) - BirdLife species factsheet". BirdLife International. Retrieved 21 October 2012.
- ^ O'connor, J, & Robertson, J (2014). Darwin's Finch Begging Intensity Does Not Honestly Signal Need in Parasitised Nests. Ethology, 228, 37.
- Jobling, James A. (2010). Helm Dictionary of Scientific Bird Names. London, UK: Christopher Helm. ISBN 978-1-4081-2501-4.