Greater Shearwater has a faster wingbeat and a darker bill, belly and cap. Audubon's Shearwater is much smaller with a dark bill. Immature gulls are superficially similar but can be separated at great distances by their different flight styles. Shearwaters fly closer to the surface of the water, often disappearing behind swells, alternating flaps with long glides on downcurved wings.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.