Loons are aquatic diving birds found in North America and Europe, where they are called divers. They are members of the genus Gavia, family family Gaviidae and order Gaviiformes.

Basal lineage
Red-throated Loon or Red-throated Diver, Gavia stellata

Black-throated lineage
Black-throated Loon, Arctic Loon or Black-throated Diver, Gavia arctica
Pacific Loon or Pacific Diver, Gavia pacifica

Black-headed lineage
Great Northern Loon, Common Loon or Great Northern Diver, Gavia immer
Yellow-billed Loon or White-billed Diver, Gavia adamsii