The Question was:
What species, age and sex is this duck?
Yep that’s what it is. If you look closely it appears to have pale fringes to wing coverts which point towards a retained juvenile feathers. Only now its nearly a year old (so ‘first summer’). Had David also seen the belly (which he didn’t) I bet it would have been white or peppered white and not uniformly dark (latter = adult). The 3 scoter species with ‘white wings’ have bill shapes which are (rather poor) ghosts of the male bill shape. So I can just make out the 2 stepped profile , which is more obvious on a male. I can also see the feathering over the bill base is quite forward over the nostrils. On Stejneger’s Scoter the profile is a virtual straight line from bill tip to in line with the eye. Flat with no steps. And the feathers envelop the bill but recede considerably over the nostrils.
What do I mean? Have a look at Ian Lewington’s illustration of the 3 taxa. Like I said, not easy but with a bit of practice…