There are three living genera of true geese: Anser, grey geese, including the domesticated goose and the Swan Goose; Chen, white geese (often included in Anser); and Branta, black geese, such as the Canada goose.
Two genera of "geese" are only tentatively placed in the Anserinae; they may belong to the shelducks or form a subfamily on their own: Cereopsis, the Cape Barren Goose, and Cnemiornis, the prehistoric New Zealand Goose.
Either these or, more probably, the goose-like Coscoroba Swan is the closest living relative of the true geese.
Fossils of true geese are hard to assign to genus; all that can be said is that their fossil record, particularly in North America, is dense and comprehensively documents many different species of true geese that have been around since about 10 million years ago in the Miocene. The aptly named Anser atavus (meaning "Great-great-great-grandfather goose") from some 12 million years ago had even more plesiomorphies in common with swans. In addition, there are some goose-like birds known from subfossil remains found on the Hawaiian Islands.
Geese are monogamous, living in permanent pairs throughout the year; however, unlike most other permanently monogamous animals, they are territorial only during the short nesting season. Paired geese are more dominant and feed more, two factors that result in more young.
Read more about this topic: Goose
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