3467 x 5200 px | 29,4 x 44 cm | 11,6 x 17,3 inches | 300dpi
Data acquisizione:
11 gennaio 2008
Altre informazioni:
a black duck standing on ice in winter 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 Drake. 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[1], Mank et al. 2004[2]). 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
