Gulls flap their wings more slowly and smoothly and lack the tube on top of the bill. Northern Fulmar's plumage does not have as much contrast and build is stockier. Manx Shearwater is smaller with faster wingbeats. Cory's Shearwater lacks the contrasting dark cap and the brownish belly and has a pale bill.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.