bitterntwisted said:
OK, I'm happy that so many people of much greater experience agree on BTD, but I am now very confused. I presumed that the bold white flank patch and clean neck line were both necessary and sufficient conditions for ID of BTD, and that the absence of these meant GND. What about the other way round? Can a GND show a clear white rear flank patch and a clear neck line? (I hope not because that puts a line through one of my best ticks last year!)
This thread has put me back to square 1. Why, for example, is the bird in Tim's 2nd sketch a BTD. I had presumed the collar in the bottom left image to be diagnostic of GND?
If these characteristics are not crucial, what is? Head shape is variable in both species. There is (small) size overlap between species and when sea-watching size comparisons may be difficult or absent. So what is a reliable characteristic?
The point about sociability is a good and very useful one, but surely it would be rather petitio principii to use this as an ID clincher.
Summary: HELP!
Graham
Hiya Graham - With regard to drawing species - that's very difficult, that's why I only attempt individuals (if you get my meaning). What I mean to say is that I observe an individual of a species and make my drawings of that single bird - that way I know what I'm seeing is honest and truthful (to me and to that particular individual, on that particular day) - I would hate to then have to make a 'standard' drawing of, say, winter plumage Great Northern Diver, because the variety across individuals can be quite striking. (Incidentally Lars Jonsson makes a similar reference in 'Birds and Light' with regard to making the 'standard' id painting of house sparrow, and he goes into some detail about the problems associated with this approach.)
But back to my drawings (or, from the sublime to the ridiculous, if you wish), it certainly helps to get to grips with Great Northern Diver winter plumage (and identifying the bird correctly) when one is lucky enough to have a handful in various plumages scattered across the bay - I was able to almost follow the moult progression of the species (in general terms) by looking from one bird, in almost full nuptial plumage, through several stages of moult and finally a bird in full winter attire. The black-throats were less distinctive (I don't know, but I assume they moult quicker??!!) and showed a more 'uniform' standard of dress, (although the residual striations along the side of the neck showed well on close observation) - except the juvs, which are distinctive because of the more finely scalloped backs and head pattern (the bird in the bottom right is a juv). But once again, it was really handy (cheers God) to have a small raft of them within 50 meters.
Returning to the original photo - if there was just a single bird in the frame I, for one, would be very reluctant to state which species I thought it to be as, from that distance, and in my experience, the two can look very similar. I think they're black-throated because of their habit, not because of any colour, pattern or jizz detail evident.
Cheers - Tim