Ring-necked Duck x Tufted Duck hybrid (with Tufted Ducks), Gavnoe (Zealand, Denmark), 18th March 2006 - copyright Rasmus Strack
(photo ID: 0907)
I'm sure Rasmus did the best that was possible given the distance from the bird, but I hope we soon have some closer shots of this hybrid before long... But even at this distance there is enough to see that this is no ordinary Tufted Duck. Lars Michael Nelson who found this bird says it showed intermediate flank colour and head markings.
This bird also showed a green head-sheen which may come as a surprise for a hybrid between two species which both normally show purplish head sheens (although they can appear green under certain light conditions). In fact this is normal for Ring-necked Duck x Tufted Duck hybrids. The head sheen colour is affected by the structure of the feathers, not just the pigments, so possibly an intermediate feather structure can reflect the light in such a way that the colour reflected is not intermediate?
Joern Lehmhus has written about Aythya hybrids in the German magazine Aves. The text is in German but non-German speakers may still be interested as it's accompanied by a photo of Ring-necked Duck x Tufted Duck hybrid. You can access the PDF of Aves 3 (2012) - the article starts on page 32 with section 5 covering this hybrid beginning at page 38 - the photo captioned "Reiherente x Ringschnabelente" (Abb. 28) is a Ring-necked Duck x Tufted Duck hybrid.
Ring-necked Duck Aythya collaris
Tufted Duck Aythya fuligula