As many of you may have already heard, the 'Black Scoter' that I reported from Morfa Bychan last Wednesday is now considered to be an aberrant Common Scoter with an extensive yellow bill (or possibly a hybrid of some sort). After seeing the bird in the field on Sunday Reg raised some concerns about the bill structure and head profile, and after sending him some of my photos this morning, we agreed that we had both seen the same bird.
While strikingly bright, the bill appears to have too much yellow for Black Scoter, extending towards the bill tip and actually looking paler nearer the tip when it should be black. The basal swelling also isn't as pronounced as on Black, although this appears to vary between individuals (see: http://www.rarebirdalert.co.uk/v2/Content/weeklyreview2012no16.aspx?s_id=433786878 for a less distinctive bulge), the combined profile and pattern of yellow on the bill don't add up for Black Scoter, which always seems to show a very clean-cut and sharply contrasting edge to the front and sides of the bulge. Looking at the bird head-on, while the yellow extends up the bill there doesn't appear to be an even bulge on either side either. The head profile is also not correct for Black (not being rounded enough) and the neck too long, both more in fitting with Common Scoter.
I'm still not certain as to whether it's an abnormal Common Scoter or a hybrid and would be very grateful to hear from anyone with experience of these.
Rob Hughes told me about a similar bird he found on Fair Isle (presumed to be a hybrid) and more details on Common x Black intergrades can be seen here: http://ardeajournal.natuurinfo.nl/ardeapdf/a60-128-129.pdf
There is also an interesting paper in BB about an adult male Common Scoter on Southport Marine Lake in 1979, which had an almost wholly yellow bill.
Thanks to everyone for their comments (and kind words!) and sincere apologies to those who travelled to see it expecting a Black Scoter. This is at least a very useful bird to learn from if nothing else (especially considering more distant birds) and it's certainly pushed up my learning curve.
Cheers and good birding,