Among the commonest hybrids are ducks in the genus Aythya.
Many have been recorded in the UK and elsewhere, but often give rise to claims of vagrants which are unfounded,
for several reasons. Firstly hybrids between closely related taxa can be very similar to one or other of the parents,
sometimes a "nail" on the end of the bill will show hybridisation, or a head shape. Secondly many hybrids are fertile,
so when they form pairs the features may be lost or more evident, dependent on the parents. Lastly Aythya
hybrids seem to have an ability to produce birds which look like other unrelated taxa, such as Lesser Scaup. Greater
and Lesser Scaup, so far as is known, rarely hybridise. The left-hand bird is a Ferruginous/Pochard hybrid, whereas
the right hand shot, superficially similar, is a different bird which was initially accepted as a Redhead.
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