Black-throated Diver. Swithland Reservoir. Watched from the dam wall. Dived every few minutes then spent a while preening. Its diving reminded me of a U-boat the way it smoothly sank underwater. Also present were 6 Scaup- immature males and females, 2 Goosander ducks, a few pairs of Goldeneye and lots of Cormorants and Great crested Grebes.