There's no doubt that hybrids are turning up in Japan, with both Common Pochard and Ferruginous Duck. Recent debate on the Kantori mailing list has highlighted how important it is to discover how much minor variation may exist within pure Baer's (turning up in Japan and elsewhere) or to what extent a diluted gene pool is responsible for the appearance of some individuals. Obviously this question is relevant to assessing the true status of the species. The discussion has focused on birds with relatively minor plumage anomalies, which makes me wonder why I see so few images of obvious hybrids compared to slightly anomalous individuals. Particularly when the risk of mixed pairing may be on the increase. Well, one reason might be that many bird photographers here won't get as excited about obvious hybrids which won't get the same level coverage as birds which are, or are very like, Baer's. Nial Moores has written this note http://www.birdskoreablog.org/?p=14726 on the Birds Korea blog with an excellent series of images mainly provided by Pete Morris.
I've seen four different Baer's and two apparently first generation hybrids in Japan. Three of the former looked good but the fourth seemingly good bird was only seen briefly at about 100m in very windy conditions which made observation difficult, certainly too difficult to assess the minutiae. At the time I didn't give this much thought as it basically looked like a Baer's and there was no opportunity to look for any subtle signs of introgression. If we need to scrutinise each out of range individual carefully to make sure it is of classic appearance or to assess how much variation is permissible then it would seem even more important that birds within the core area are given the same level of attention. It might also be instructive pay closer attention to Ferruginous Ducks in areas of potential sympatry too.
I've only seen two obvious hybrids (presumably F1), both on the Yodogawa in Osaka, Japan. Interestingly these two birds were in close association when I first found them on 29 December 2001 then again on 11 January 2002. Even more interesting, one suggested a Ferruginous hybrid while the other was clearly a Common Pochard hybrid. Perhaps their common Baer's heritage drew them together and though both were males the implication is that hybrids from different progenitor pairings could be drawn together producing even more mysterious appearances than back-crossing. Unlikely as it may seem that hybrids would find each other on the breeding grounds these two males show there is an affinity and the possibility exists.
The birds were distant and in those days I used to hand-hold an old camcorder against the telescope to get a closer look at distant gulls and ducks. The resulting video grabs are very low quality showing neither the fine detail nor with the faithful colour rendition which would be needed to assess the birds had they not been such obvious hybrids. However these images are adequate to give a general idea of their overall appearance. So apologies for the dreadful quality but they may nevertheless be of interest.
As it isn't possible to determine the colours from these grabs I'll just add that the larger of the two had pale grey flanks and darker brownish-grey, vermiculated upper parts becoming darker and more uniform aft. The head could look Baer's green or reddish-brown depending on the angle and light and the breast dark brown. The vent was black with white restricted to the undertail coverts. The smaller bird was brown above with paler brownish flanks. The head and breast were rich brown and the vent was conspicuously white bordered with blackish-brown. Both had pale fore flanks behind the breast recalling Baer's. Both also had very pale eyes.
The first image shows both birds together with Common Pochards followed by two images of the Ferruginous-type bird and finally several of the Common Pochard hybrid which did come closer on the second occasion I saw them.
Both birds together with Common Pochards.
Two grabs of the smaller, more Ferruginous-like of the two.
The bird looked very green-headed in the part of the video this final grab was taken from.
As a matter of interest the most recent Baer's I saw was a bird in Osaka in December 2013 which returned to the same site, at least briefly, in December 2014. When I first saw it on 1 Dec it showed a distinct brownish tinge to the cheeks but seeing it again on 21 Dec the head was wholly green.
21 Dec 2013, the bird looks far more convincing with no hint of brown remaining, the head was glossy green.