The identity of this bird, which was first spotted in the sea off Cardiff Bay’s Barrage on Wednesday the 29th, caused a lot of discussion among local birders, which made for an excellent learning experience for me, as I’d only ever seen this species once before. It seems that Black-throated divers (known as Arctic loons in North America) and Red-throated divers (also known as Red-throated loons) can be tricky to tell apart when they’re not in breeding plumage.

The white flank patch on this bird would normally indicate Black-throated but that species, when not in full breeding plumage, almost always has a throat that is mostly black around the back but has a white strip down the front (see this image on Cornell’s All About Birds website). The bills of these species can also be diagnostic; the bill of the Red-throated diver often looks slightly more upswept at the tip, while the Black-throated’s bill looks heavier and straighter; though some birders queried the shape of this bird’s bill, it seems to be within acceptable parameters.

The final decision was that this is a Red-throated diver (Gavia stellata), an adult bird that is still moulting from its stunning breeding plumage into its full winter plumage. Apparently, the white speckling/fringes to the back feathers indicate it’s an adult bird not a juvenile, which would usually have a black back. Cornell’s Macaulay Library has a photo of a similar-looking bird, a much clearer photo that my camera was able to produce at distance and in poor light.