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Of course, in the southern hemisphere, the directions are reversed, but there is less land area in the far south to support long-distance migration. [19] The primary motivation for migration appears to be food; for example, some hummingbirds choose not to migrate if fed through the winter. [20]
A few hundred species migrate long distances, in some cases of thousands of kilometres. About 120 species of fish, including several species of salmon, migrate between saltwater and freshwater (they are 'diadromous'). [22] [23] Forage fish such as herring and capelin migrate around substantial parts of the North Atlantic ocean.
The mallard has hybridised with more than 40 species in the wild, and an additional 20 species in captivity, [13] though fertile hybrids typically have two Anas parents. [14] Mallards and their domestic conspecifics are fully interfertile; many wild mallard populations in North America contain significant amounts of domestic mallard DNA. [15] [16]
Most hummingbirds migrate in the winter to Central America or Mexico, but some hummingbirds spend the winter on the Gulf Coast and may be found in South Texas and South Louisiana during mild winters.
The Eurasian spoonbill (Platalea leucorodia), or common spoonbill, is a wading bird of the ibis and spoonbill family Threskiornithidae, native to Europe, Africa and Asia.The species is partially migratory with the more northerly breeding populations mostly migrating south for the winter.
Mallards, both domestic and wild in particular are notorious hybridizers, know to frequently hybridize within the mallard complex and even outside of Anas. The willingness to hybridize has concerned conservationists as the gene flow from the mallard may pollute pure populations of more vulnerable species such as the Mexican duck. [ 2 ]
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In flight, the lack of a white border to the speculum is a key difference. The American black duck is darker than most mottled ducks, and its wing-patch is more purple than blue. The behaviour and voice are the same as the mallard. Mottled ducks feed by dabbling in shallow water, and grazing on land.