Green-winged Teal
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| iGreen-winged Teal | ||||||||||||||
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 A pair, male in nuprial plumage at rear
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| Anas crecca Gmelin, 1789 |
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 Distribution the Green-winged Teal in North America and the Common Teal in Europe and Asia
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Anas crecca carolinensis |
The Green-winged Teal (GWT, Anas carolinensis) is a common and widespread duck which breeds in the northern areas of North America except on the Aleutian Islands. It was considered conspecific with the Common Teal for some time, and the issue is still being reviewed by the AOU [1]; based on this the IUCN and BirdLife International (BirdLife International, 2004) do not accept it as a separate species at present. However, nearly all other authorities consider it distinct nowadays, based on behavioral (Laurie-Ahlberg & McKinney 1979), morphological (Livezey, 1991), and molecular (Johnson & Sorenson 1999) evidence (discussed by Sangster et al., 2002).
This dabbling duck is strongly migratory and winters far south of its breeding range. It is highly gregarious outside of the breeding season and will form large flocks. In flight, the fast, twisting flocks resemble waders.
This is the smallest North American dabbling duck. The breeding male has grey flanks and back, with a yellow rear end and a white-edged green speculum, obvious in flight or at rest. It has a chestnut head with a green eye patch. It is distinguished from drake Common Teal (the Eurasian relative of this bird) by a vertical white stripe on side of breast, the lack of both a horizontal white scapular stripe and the lack of thin buff lines on its head.
The females are light brown, with plumage much like a female Mallard. They can be distinguished from most ducks on size and shape, and the speculum. Separation from female Common Teal is problematic.
In non-breeding (eclipse) plumage, the drake looks more like the female.
It is a common duck of sheltered wetlands, such as taiga bogs, and usually feeds by dabbling for plant food or grazing. It nests on the ground, near water and under cover. While its conservation status is not evaluated by IUCN at present due to non-recognition of the taxon, it is plentiful enough to make it a species of Least Concern if it were; it is far more plentiful than the Common Teal (Carboneras, 1992). It can be seen in vast numbers in the Marismas Nacionales of western Mexico, a main wintering area.
This is a noisy species. The male has a clear whistle, whereas the female has a feeble "quack".
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[edit] Relationships
mtDNA data-wise, this species is more closely related to the Speckled Teal than to the Common Teal (Johnson & Sorenson 1999). This would require that sexual dimorphism either was lost in the Speckled Teal or that it evolved in near-identical forms in the Green-winged and Common Teal after the divergence of the Green-winged and Speckled Teal lineages.
Both hypotheses seem rather spurious initially, with the GWT and Common Teal's male nuptial plumage being unique and very complex, and the tendency to gain, not lose, strong sexual dimorphism overwhelming in the dabbling ducks. An alternative explanation given the high frequency of hybridisation in ducks (Carboneras, 1992) would be that the mtDNA of the present-day GWT is originally derived from Spotted Teal females by introgression and thus the molecular data gives a misleading picture of the species' true relationships. This is supported by the observation that in Mallard × American Black Duck hybrids, females of both taxa prefer the sexually dimorphic mallard drakes over the dull-plumaged Black Duck drakes (Brodsky et al., 1988, but see also Rhymer et al., 1994); that the GWT is in some aspects - such as the less contrasting nuptial plumage - intermediate between the Common and Speckled Teal is also interesting to note.
Alternatively, the Common Teal might actually be derived from the GWT, with the molecular difference being due to genetic drift or a founder effect in the latter and possibly Speckled Teal introgression in the former. The three teals certainly belong to a superspecies in the teals ("Nettion"); the ancestors of this group were most likely sexually monomorphic Southern Hemisphere forms (Johnson & Sorenson, 1999; note that their "African" distribution includes Bernier's Teal taxa which like many Madagascar taxa is of Indo-Australian origin).
Still another possibility - and perhaps the most likely one[1] -, is that the American lineage is derived from stray Common Teals, with the founder effect/genetic drift and/or hybrid introgression phenomena applying as above, only in the reverse direction for the former two. Still, this would require loss of sexual dimorphism in the ancestors of the Speckled Teal, but while extremely rare in dabbling ducks, it is not per se impossible.
The close Speckled-GWT relationship suggested by mtDNA data could of course still apply to the taxa in general, not just to sequences in 2 maternally inherited genes in a few individual ducks (for which it without doubt does apply), but the overall failure of Johnson & Sorenson to seriously take hybridization into account and their small sample sizes and obsolete conceptions of Indian Ocean biogeography do not help at all to resolve the issue[2], but in 1999, the methodology and interpretation were reasonable enough and in fact, the study was pioneering in many respects due to dense taxon-level sampling and still represents one of the default references for interpreting the pyhlogeny of the genus.
It is worthy of note that the post-copulatory displays of the Common and GWT are identical, but those of the Speckled Teal have some additional elements (Johnson et al., 2000).
All in all, a firm conclusion cannot be reached at present beyond a tentative rejection of the phylogeny suggested by the mtDNA data. Nuclear DNA sequence information is required, but may not be sufficient, to resolve the puzzling relationships in the crecca-carolinensis-flavirostris complex of teals.
[edit] References
- BirdLife International (2004). Anas crecca. 2006 IUCN Red List of Threatened Species. IUCN 2006. Retrieved on 11 May 2006. Database entry includes a brief justification of why this species is of least concern, and the criteria used
- Brodsky, L. M.; Ankney, C. D. & Dennis, D. G. (1988): The influence of male dominance on social interactions in Black Ducks and Mallards. Animal Behaviour 36: 1371-1378. DOI:10.1016/S0003-3472(88)80206-9 (HTML abstract)
- Carboneras, Carles (1992): Family Anatidae (Ducks, Geese and Swans). In: del Hoyo, Josep; Elliott, Andrew & Sargatal, Jordi (editors): Handbook of Birds of the World, Volume 1: Ostrich to Ducks: 536-629. Lynx Edicions, Barcelona. ISBN 84-87334-10-5
- Johnson, Kevin P. & Sorenson, Michael D. (1999): Phylogeny and biogeography of dabbling ducks (genus Anas): a comparison of molecular and morphological evidence. Auk 116(3): 792–805. PDF fulltext
- Johnson, Kevin P. McKinney, Frank; Wilson, Robert & Sorenson, Michael D. (2000): The evolution of postcopulatory displays in dabbling ducks (Anatini): a phylogenetic perspective. Animal Behaviour 59(5): 953–963 PDF fulltext
- Laurie-Ahlberg, C. C. & McKinney, F. (1979): The nod-swim display of male Green-winged Teal (Anas crecca). Animal Behaviour 27: 165–172. DOI:10.1016/0003-3472(79)90136-2 (HTML abstract)
- Livezey, B. C. (1991): A phylogenetic analysis and classification of recent dabbling ducks (Tribe Anatini) based on comparative morphology. Auk 108(3): 471–507. PDF fulltext
- Rhymer, Judith M.; Williams, Murray J. & Braun, Michael J. (1994). Mitochondrial analysis of gene flow between New Zealand Mallards (Anas platyrhynchos) and Grey Ducks (A. superciliosa). Auk 111(4): 970–978. PDF fulltext
- Sangster, George; Knox, Alan G.; Helbig, Andreas J. & Parkin, David T. (2002): Taxonomic recommendations for European birds. Ibis 144(1): 153–159. DOI:10.1046/j.0019-1019.2001.00026.x PDF fulltext
[edit] Footnotes
- ^ The trans-Beringia populations are of the Common Teal type, which would be odd if the lineage is of South American origin
- ^ Note that their study dates from a time when molecular phylogeny was still in its infancy; it would be considered rather sloppy science by today's standards; the hypothesis discussed in this section rests on a sample size of 1 (one) individual of the GWT.
