Audubon's shearwater ill-resolved clade also including such species as the little shearwater, the Manx shearwater ("P. puffinus") or the black-vented shearwater ("P. opisthomelas"). The little-known Heinroth's shearwater ("P. heinrothi") is sometimes considered a subspecies of either Audubon's or the little shearwater. Though it is likely to be another member of that close-knit group, its actual relationships remain uncertain due to lack of specimens. Audubon's shearwater itself has around 10 subspecies. Several have at one time or another been suggested to constitute separate species. For example, the Galápagos Islands population has turned out to be a very distinct species, the Galápagos shearwater ("P.
Procellariidae been implicated in (but not proved to have caused) the expansion in range of the northern fulmar in the Atlantic. Three of the six prion species have bills filled with lamellae, which act as filters to sift zooplankton from the water. Water is forced through the lamellae and small prey items are collected. This technique is often used in conjunction with a method known as hydroplaning where the bird dips its bill beneath the surface and propels itself forward with wings and feet as if walking on the water. Many of the shearwaters in the genus "Puffinus" and "Ardenna" are