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 dark throat separates it from the Red-throated Loon. In basic plumage it has much more contrast between the dark nape and white throat than the other species. In all plumages it has a straighter, not upturned bill unlike the Red-throated and Yellow-billed Loons while the Common Loon has a larger bill. The Pacific Loon was formerly considered conspecific with the Arctic Loon. The Arctic Loon is very similar in appearance, has a green throat patch in breeding plumage, a larger bill and whiter flanks.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.