I've always enjoyed setting the questions at the KOS quiz night, and since the next one is 11 months away I'll probably be able to use this picture, as no-one will remember it by next December. The main problem is I can't think of any way I can show that it really is what I say it is. It looks so similar to all small grebes when they dive!
I was interested to find out that there was a Slavonian Grebe in the dock's at Dover, by the De Bradlei Wharf. Fortunately the news had found its way onto "Bird News" as I hadn't had a call. I believe that this is another of Mark Chidwick's finds, so well done to him. Why is it called a Slavonian Grebe? The best I can find is that the first reference was in "Montagu 1802" who referred to a Horned or Sclavonian Grebe, with the remark that Dr Latham says if it is found in Sclavonia, a reference to it's northern Russian home of European birds. However I can only find a reference to Sclovonia as a "State" that is in the northern Blakans, then northern part of Croatia, bordering Hungary, and nowhere near Russia! Anyway by 1883 it was in use by the BOU and later it changed to Slavonian.
At this time of year there's not much to justify it's American name of Horned Grebe, but come the spring the golden head tufts give the impression of horns from the front view.
At this time of year, at least from a distance, the impression one has is of a small black and white grebe. Once the contrasting plumage has eliminated Little Grebe from the possibilities the only other contender is Black-necked (Eared in the USA) Grebe. Given the fantastic views this bird gave all the distinguishing features were evident.
The head is rather angular and peaked at the back, and the bill is dark and straight, with a pale tip, not up-tilted as in a Black-necked. The white cheek is much cleaner and cuts off just below the eye giving a capped appearance.
In many ways a Slavonian Grebe looks like a small version of a Great-crested Grebe, rarely showing the "cut-off" back-end profile of a Little Grebe that is often shared by a Black-necked Grebe.
One of the most obvious marks is the white spot between the base of the bill and the eye. It also shows a slight light grey collar, whilw a black necked shows a much dark collar. Apart from other grebes it should be remembered that female/immature Smew can look a similar pattern from a distance. There are a few more pictures here.