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BTD has a much less robust bill and head, and shows a distinct white patch on the flanks. In winter on the sea (I assume that is where it was, though your location map is in the middle of Aberdeen, not Orkney - you might edit that so it is correct) in N Scotland GND is far commoner.
recording wildlife with The Recorder's Year on www.hbrg.org.uk/TRY.html.
Divers in non breeding plumages are tricky to ID - especially when sightings are often much more distant than this with the bird bobbing in and out of sight in the waves!
For those I-spotters who find this a difficult group to identify, it would be helpful, Ardea, if you could comment on what features lead you to determine this is Gavia immer.
Thanks Syrphus - your comment was posted while I was in the middle of writing mine so your answer was ready and waiting when I uploaded it!
The bird in the picture appears to be swimming very low in the water with its flanks largely submerged. Would the white necessarily show in this case? (I am not querying the ID - just trying to get to grips with the finer points of diver spotting!).
>>>Would the white necessarily show in this case?<<<
Possibly not at the precise situation in the picture, but it is a very good feature generally - and the bird would not always have been that low. My comment was more for general application in the field than for the picture we have here. If you Google for images of BTD you will see exactly what I mean (e.g. http://blog.dartfordwarbler.com/2008_02_01_archive.html), and then do the same for GND.
Wasn't sure if it was a GND or a BTD so cheers for clearing that up. Also updated the map location, if still not working it was seen off the SW coast of Papa Westray in Orkney
One or two other pointers that might be useful:
1. GND has relatively sharp angle at front of crown, usually fairly noticeable (can be seen on this bird) except immediately prior to diving. BTD has more smoothly-curved crown.
2. BTD lacks the beginnings of a collar at the base of the neck.
3. White cheeks are more clearly / neatly demarcated from dark 'upper face' in BTD than GND, giving it a 'neater' appearance.
4. BTD has silky grey nape; GND is invariably brown-grey in this area.
Lat/Lng: 59.332, -2.9128
OS grid ref: HY481498