The Black-throated Diver has a similar distribution to the Red-throated Diver (although it's not generally found in Iceland!), however in North America/Canada and eastern Siberia the very similar Pacific Loon (Gavia pacifica) occurs instead. The Black-throated Diver's preferred breeding habitat is clear lakes or calm sea bays. In summer plumage, it's easily told apart from Red-throated Diver, but in winter when they look quite similar, the Black-throated Diver has less extensive white on its throat, a 'full-chested' appearance, more or less uniform black on its back and often a peaked forehead (as in the photo below). I have never seen a Black-throated Diver in summer plumage - maybe that should be next on my list.