Loons

Five species, all of them in the Northern Hemisphere, and known as divers in Europe. Foot-propelled pursuit divers with legs set so far back that walking is barely possible, and most species cannot take off from land at all. The bones are dense rather than air-filled, which reduces buoyancy. They breed on northern lakes and winter at sea, and the two plumages are so different that the same bird looks like two species.