Sure it's got a great big yellow knob on the bill, at least the adult males have. All the Black Scoters recorded so far in Britain and Western Europe have been adult males. Why just adult males what about other ages? They must be occurring and must be overlooked as the differences are 'subtle' (to quote someone else).
As I wrote in my post yesterday I noticed the frequent and prolonged opening of the bill during display of the adult male Black Scoter currently off Bamburgh. One or two other observers have noted and commented on it again today. I did a little reading and came across a paper on the 'Acoustic Differences Between Nigra and Americana' (see here).
It makes some interesting points including ' The calls of americana are more protracted and often involve a rise in pitch (either at the start of the call, in the middle of the note, or towards the end) which is not observed in nigra.'
Also ' ...on average the call of nigra lasts for just 0.1 seconds whilst americana averages 0.7 seconds' On a spectrogram that highlights a huge visual difference and could well be the reason behind the more obvious bill opening observed in this individual.
Is it only adult males that utter courtship calls in Spring? Might it be possible to focus in on a 1st-winter male that is courtship calling amongst a group and use the extended bill opening as a way of indicating an individual that warrants further attention?
Edit: That's always assuming 1st-winter males get involved in courtship of course!