Ring-necked Duck x Ferruginous Duck hybrid - copyright Joern Lehmhus
(illus. ID: 0653)
Joern's drawing is based on photos of two birds from the web, one from England and the other without a location specified. One of the two birds had totally black undertail coverts. He has written about these in the German magazine Aves. The text is in German but if you can read German it's worth accessing the PDF of Aves 3 (2012) - the article starts on page 32 with section 5 covering this hybrid beginning at page 38.
This next bird is rather different, chiefly in having darker flanks (more like Ferruginous Duck). It also lacks much evidence of Ring-necked Duck in its head shape. It wasn't submitted to us as a Ring-necked Duck x Ferruginous Duck hybrid and indeed that may not be the correct ID. But to my eyes at least it has a prominent pale subterminal band on the bill and a hint of the white surround to the base of a bill (clearest in the head-on photo) which together are suggestive of Ring-necked Duck influence.
I wonder if the situation with this hybrid is analagous to that of Ferruginous Duck x Tufted Duck in which most males seem to have pale flanks (as in Joern's illustration of Ring-necked Duck x Ferruginous Duck, above) but some reportedly show darker more Ferruginous Duck-like flanks. These birds are also more Ferruginous Duck-like in other respects (e.g. head colour). Perhaps there are two types of Ring-necked Duck x Ferruginous Duck hybrids also, the more frequent one showing paler flanks and clear evidence of Ring-necked Duck in head shape and the other showing darker flanks and a less obvious head shape? At the moment this is simply speculation - more evidence is needed before we can draw conclusions. Here is the bird anyway, let us know if you have any thoughts about it.
possible Ring-necked Duck x Ferruginous Duck hybrid, Throckmorton Lagoons (Worcestershire, UK), 9th December 2006 - copyright Andy Warr
(photo IDs: 2433-2435)
Ring-necked Duck Aythya collaris
Ferruginous Duck Aythya nyroca