Great Northern Diver in Chatham Docks

Easily spotted in the end, this bird showed a number of confirmatory characteristics.

There was a dark half collar at the base of the neck although this was variably obvious according to the bird's activity and its angle to the viewer. The flattened head, almost concave is visible here. The bill was held horizontal and was ivory-coloured except a narrow dark triangle along the top ridge of the upper section, referred to as the culmen.

Note the lack of a clear white flash on the flank, as would be expected in a juvenile Black-throated Diver.

In the next photograph, you can see the half collar again.

In the next photograph the markings on the back are nice and clear, possibly indicating the white fringed feathers of a juvenile bird. 

The bird was quite active and had regular periods of "splashing about" sometimes apparently with the legs, but with the wings also more or less ruffled. Incidentally, here the head outline looks quite concave.

At other times it is clearly the wings themselves being used to disturb the water surface.

After one of these episodes, the diver dropped its head below the surface, perhaps snorkelling, and then did an upright "flapping" display. Divers and Cormorants do this, Grebes never do:

Here is the same bird doing the same again!

The wing tips often "stuck up" in this bird.