Photographed by Cin-Ty, Galveston TX today:
having been involved with a lot of the birds in NW Scotland in recent years I would say that this is a Yellow billed Loon. All the notes fit and are backed up by the photos.
by the way the photos are far better than the ones we often get on here!
Many thanks Martin. The photos of your moulting birds in Scotland were very helpful. There seem to be few photos of this species in this transitional plumage on the web. Welcome comments from others with experience of this species. In meantime, Cin-Ty will try and get better photos.