The White-backed Duck (Thalassornis leuconotus) is a waterbird of the family Anatidae. It is distinct from all other ducks, but most closely related to the whistling ducks in the subfamily Dendrocygninae, though also showing some similarities to the stiff-tailed ducks in the subfamily Oxyurinae (family Anatidae). It is the only member of the genus Thalassornis, (Eyton, 1838).
These birds are well adapted for diving. On occasions they have been observed to stay under water for up to half a minute. They search especially for the bulbs of waterlilies.
White-backed Ducks live in southern Africa, especially between Senegal and Chad in the west and Ethiopia and South Africa in the east. Their habitat consists of lakes, ponds, swamps and marshes where they are well camouflaged against predators.
There are two subspecies, Thalassornis leuconotus leuconotus and Thalassornis leuconotus insularis. The latter lives entirely on Madagascar and is considered to be endangered by hunting, habitat loss and the introduction of competing exotic species.
Thalassornis resembles Dendrocygna in the form of the pubis, but otherwise is convergent to Biziura, especially in its relative proportions, which indicate a greater degree of adaptation for diving than in Dendrocygna.
The lower jaw of Thalassornis resembles that of Dendrocygna because the posterior edge of the retroarticular process is straight, while in the Oxyurini (except Biziura) it is rounded.
Since Thalassornis is the only known member of the Anserinae which has become highly specialized fo.r diving, it is possible that more such types failed to develop because this adaptive zone was being actively entered by several groups of true ducks (Anatinae), which were perhaps better fitted to exploit this way of life.
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