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American Black Duck

The American Black Duck (Anas rubripes) is a large dabbling duck.

The adult male has a yellow bill, a dark body, lighter head and neck, orange legs and dark eyes. The adult female has a similar appearance. Both sexes have a shiny purple-blue wing patch, which is not bordered with white as with the Mallard. The behaviour and voice are the same as for Mallard.

Their breeding habitat is lakes, ponds, rivers, marshes and other aquatic environments in eastern Canada including the Great Lakes, and the Adirondacks in the U.S. Black Ducks interbreed regularly and extensively with Mallard ducks, to which they are closely related. Some authorities even consider the Black Duck to be a dark-plumaged subspecies of the Mallard, not a separate species at all; this is in error as the extent of hybridization alone is not a valid means to delimitate Anas species (Mank et al. 2004).

In the past, Black Ducks and Mallards were separated by habitat preference, with the dark-plumaged Black Ducks having a selective advantage in shaded forest pools in eastern North America, and the lighter plumaged Mallards in the brighter, more open prairie and plains lakes. In recent times, deforestation in the east, and tree planting on the plains, has broken down this habitat separation, leading to the high levels of hybridisation now seen.(Johnsgard 1967) Indeed, American Black Ducks and local mallards are now very hard to distinguish by means of microsatellite comparisons, even if many specimens are sampled (Avise et al. 1990, Mank et al. 2004).

The hybrids cannot be readily distinguished in the field and consequently, much of the species' hybridization dynamics remains unknown. It has been revealed in captivity studies, however, that the hybrids follow Haldane's Rule, with hybrid females often dying before they reach sexual maturity (Kirby et al. 2004); this underscores the case for the American Black Duck being a distinct species.

This species is partially migratory and many winter in the east-central United States, especially coastal areas; some remain year-round in the Great Lakes region. These birds feed by dabbling in shallow water, and grazing on land. They mainly eat plants, but also some molluscs and aquatic insects. The eggs are a greenish buff color. They lay from 6-14 eggs, and hatch in an average of 30 days.

This duck is a rare vagrant to Great Britain, where, over the years, several birds have settled in and bred with the local Mallards. The resulting hybrids can present considerable identification difficulties.

Status

The Black Duck has long been valued as a game bird, being quite wary and fast on the wing. Although this is a species of least concern, it is slowly declining due to overhunting and habitat destruction. Some conservationists consider the hybridization and competition with the Mallard an additional source of concern, should this decline continue (Rhymer & Simberloff 1996, Rhymer 2006). It should be noted that the hybridization itself is not the major problem; natural selection will see to that the best-adapted individuals still have the most offspring. But the reduced viability of female hybrids will cause many broods to fail in the long run as the offspring die before reproducing themselves. While this is not a problem in the plentiful mallard, it will place an additional strain on the poopo!

References

  • Avise, John C.; Ankney, C. Davison & Nelson, William S. (1990): Mitochondrial Gene Trees and the Evolutionary Relationship of Mallard and Black Ducks. Evolution 44(4): 1109-1119. (HTML abstract and first page image)
  • Database entry includes justification for why this species is of least concern
  • Johnsgard, Paul A. (1967): Sympatry Changes and Hybridization Incidence in Mallards and Black Ducks. American Midland Naturalist 77(1): 51-63. (HTML abstract and first page image)
  • Kirby, Ronald E.; Sargeant, Glen A. & Shutler, Dave (2004): Haldane's rule and American black duck × mallard hybridization. Canadian Journal of Zoology 82(11): 1827–1831. (HTML abstract)
  • Mank, Judith E.; Carlson, John E. & Brittingham, Margaret C. (2004): A century of hybridization: Decreasing genetic distance between American black ducks and mallards. Conservation Genetics 5(3): 395–403. (HTML abstract)
  • Rhymer, Judith M. (2006): Extinction by hybridization and introgression in anatine ducks. Acta Zoologica Sinica 52(Supplement): 583–585. PDF fulltext
  • Rhymer, Judith M. & Simberloff, Daniel (1996): Extinction by hybridization and introgression. Annu. Rev. Ecol. Syst. 27: 83-109. (HTML abstract)

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