|Common shelduck (Tadorna tadorna)|
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The Tadorninae is the shelduck-sheldgoose subfamily of the Anatidae, the biological family that includes the ducks and most duck-like waterfowl such as the geese and swans.
This group is largely tropical or Southern Hemisphere in distribution, with only two species, the common shelduck and the ruddy shelduck breeding in northern temperate regions, though the crested shelduck (presumed extinct) was also a northern species.
Most of these species have a distinctive plumage, but there is no pattern as to whether the sexes are alike, even within a single genus.
Following the review of Livezey (1986), several species formerly classified as aberrant dabbling ducks or as "perching ducks" were placed in the Tadorninae. mtDNA sequence analyses cast doubt on the allocation of several genera; many supposed dabbling ducks and one peculiar goose may more correctly belong here, while some genera believed to be close to shelducks appear to have different relationships altogether.
The available data indicates that the Tadorninae are indeed, as their appearance suggests, somewhat intermediate between geese and dabbling ducks, but the molecular data suggests they are not the only lineage to evolve towards a more duck-like morphology, with the diving ducks and seaducks being more distant.
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