After this, they shift to seeds and other plant food. Breeding The breeding habitat includes alkaline marshes, acid bogs, lakes, ponds, rivers, marshes, brackish marshes and the margins of estuaries and other aquatic environments in northern Saskatchewan, Manitoba, across Ontario, Quebec and the Atlantic Canadian Provinces, plus the Great Lakes and the Adirondacks in the United States. It is partially migratory, and many winter in the east-central United States, especially coastal areas; some remain year-round in the Great Lakes region. This duck is a rare vagrant to Great Britain and Ireland, where over the years several birds have settled in and bred with the local mallard. The resulting hybrid can present considerable identification difficulties. Nest sites are well-concealed on the ground, often in uplands. Egg clutches have six to fourteen oval eggs, which have smooth shells and come in varied shades of white and buff green. On average, they measure long, wide and weigh . Hatching takes 30 days on average. The incubation period varies, but usually takes 25 to 26 days. Both sexes share duties, although the male usually defends the territory until the female reaches the middle of her incubation period. It takes about six weeks to fledge. Once the eggs hatch, the hen leads the brood to rearing areas with abundant invertebrates and vegetation. The American black duck interbreeds regularly and extensively with the mallard, to which it is closely related. Some authorities even consider the black duck to be a subspecies of the mallard instead of a separate species. Mank et al. argue that this is in error as the extent of hybridization alone is not a valid means to delimitate Anas species. It has been proposed that the American black duck and the mallard were formerly separated by habitat preference, with the American black duck's dark plumage giving it a selective advantage in shaded forest pools in eastern North America, and the mallard's lighter plumage giving it an advantage in the brighter, more open prairie and plains lakes. According to this view, recent deforestation in the east and tree planting on the plains has broken down this habitat separation, leading to the high levels of hybridization now observed. However, rates of past hybridization are unknown in this and most other avian hybrid zones, and it is merely presumed in the case of the American black duck that past hybridization rates were lower than those seen