Medium ground finch

The medium ground finch (Geospiza fortis) is a species of bird in the family Thraupidae. It is endemic to the Galapagos Islands. Its natural habitats are subtropical or tropical dry forests and subtropical or tropical dry shrubland. One of Darwin's finches, the species was the first which scientists have observed evolving in real-time.

Medium ground finch
Male
Female
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Aves
Order: Passeriformes
Family: Thraupidae
Genus: Geospiza
Species:
G. fortis
Binomial name
Geospiza fortis
Gould, 1837

The population of medium ground finches has been experiencing inbreeding depression due to small population numbers. Inbreeding depression occurs when there is a decrease in fitness due to individuals mating with genetic relatives. Typically, this leads to a loss of genetic diversity and a reduction in heterozygosity.[2]

Description

Like the other members of its genus, the medium ground finch is strongly sexually dimorphic; the female's plumage is brown and streaky,[3] while male's is solid black, with white tips to the undertail coverts. The bird measures 12.5 cm (4.9 in) in length—which falls between the lengths of the small and large ground finches.[4][nb 1] The bill of this species is quite variable in size, though the length of the upper mandible is always greater than the depth of the bill at its base.[4] The wing shape, on average, seems to change with ecological shifts. Different selective pressures act on the wing shape of the finches, such as natural and sexual selection. The males have shorter, rounder wings, which help with maneuvering around a female during sexual displays.[6]

Evolution

In 1977, a severe drought reduced the supply of seeds in the Galapágos. The finch, which normally preferred small and soft seeds, was forced to turn to harder, larger seeds. This strong selective pressure favoring larger beaks, coupled with the high heritability of traits relating to beak size in finches, caused the medium ground finch population to experience evolution by natural selection, leading to an increase in average beak size in the subsequent generation.[7]

Evidence of evolution through character displacement has been found in a population of medium ground finches on the Galápagos island of Daphne Major. During a drought in 2004, overlap in the diets of the medium ground finch population and a recently settled population of large ground finches (Geospiza magnirostris) led to competition for a limited supply of seeds on which the medium ground finch population normally fed.[8] Because the large ground finches were able to out-compete the medium ground finches for these seeds due to both a larger beak and body size, the medium ground finch population experienced a strong selective pressure against large beaks to avoid competition, ultimately leading to dramatic evolutionary change favoring smaller beaks in the subsequent generation.[8]

Habitat and range

Endemic to the Galápagos,[9] the medium ground finch is found on ten islands: Baltra, Floreana, Isabela, Fernandina, Seymour, Pinzón, San Cristóbal, Santa Cruz, Santa Fé, and Santiago.[10]

Behavior

Feeding

The medium ground finch feeds primarily on seeds, although it is also known to eat flowers, buds, and young leaves, and the occasional insect. It forages either on the ground or in low vegetation.[3]

Parasites

The medium ground finch has been under parasitism of Philornis downsi, as well as the avian pox virus (Poxvirus avium). Although outbreaks of the virus have been historically rare, in 2008 there was an outbreak that showed to be present in 50% of the finches tested. As a result, the finches have developed antibodies to fight specific invasive parasites.[11] The finches with the highest amount of antibodies tend to have the highest fitness, and therefore produce more viable offspring.

Philornis downsi

Using a new mathematical model, a 2015 study, by Dr Jennifer Koop of the University of Massachusetts Dartmouth, suggests the population of 270,000 birds on Santa Cruz may become extinct in 50 years. The flies lay eggs in the nest including in the nestlings nostrils. The larvae feed on living tissue and in worst cases can perforate the bill. Possible solutions include the introduction of parasitic wasps which would lay eggs on the larva, or cotton wool treated with a pesticide which the adult birds would use when constructing the nest.[12]

Note for the Finches
  1. 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.[5]

References
  1. BirdLife International (2012). "Geospiza fortis". IUCN Red List of Threatened Species. 2012. Retrieved 16 August 2012.CS1 maint: ref=harv (link)
  2. Markert, J, & Grant, P (2004). Neutral Locus Heterozygosity, Inbreeding, and Survival in Darwin’s Ground Finches (Geospiza fortis and G. scandens). Heredity, 306, 15.
  3. Scott, Thomas, ed. (1996). Concise Encyclopedia Biology. Berlin, Germany: Walter de Gruyter. p. 510. ISBN 978-3-11-010661-9.
  4. Swash & Still (2006), p. 102.
  5. Cramp, Stanley, ed. (1977). Handbook of the Birds of Europe, the Middle East and North Africa: Birds of the Western Palearctic, Volume 1, Ostrich to Ducks. Oxford University Press. p. 3. ISBN 978-0-19-857358-6.
  6. Vanhooydonck, B, & Herrel, A (2009). Wing Shape Variation in the Medium Ground Finch (Geospiza fortis): An Ecomorphological Approach. Biological Journal of the Linnean Society, 129, 38.
  7. Grant, P. R.; Grant, B. R. (1995). "Predicting microevolutionary responses to directional selection on heritable variation". Evolution. 49: 241–251.
  8. Grant, P. R.; Grant, B. R. (2006). "Evolution of character displacement in Darwin's finches". Science. 313: 224–226.
  9. Swash & Still (2006), p. 32.
  10. Swash & Still (2006), p. 152.
  11. Huber, S, & Owen, J (2010). Ecoimmunity in Darwin's Finches: Invasive Parasites Trigger Acquired Immunity in the Medium Ground Finch (Geospiza fortis). Public Library of Science, 5, 1-6.
  12. McGrath, Mike. "Growing parasite threat to finches made famous by Darwin". BBC News. Retrieved 20 December 2015.

Sources

Swash, Andy; Still, Rob (2006). Birds, Mammals and Reptiles of the Galapagos Islands: An Identification Guide. New Haven, CT, USA: Yale University Press. ISBN 978-0-300-11532-1.

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