Cormorants have hooked bills. Western, Clark's, and Red-necked grebes have thinner variably yellow bills and show white in the wings in flight. Other species of loons can be quite similar in appearance. In alternate plumage the paler gray head differentiates it from the darker-headed Common and Yellow-billed loons while the red throat separates it from the Pacific Loon. In basic plumage it has much less contrast between the dark nape and white throat than the Pacific Loon, lacks the jagged border present on the Common Loon, and lacks the dark auricular of the Yellow-billed Loon. In all plumages the thin, upturned bill separates it from Pacific and Common loons while the bill is darker and smaller than that of the Yellow-billed Loon.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.