The family Procellariidae is the main radiation of medium-sized true petrels, characterised by united nostrils with medium septum, and a long outer functional primary feather. It is dominant in the Southern Oceans, but not so in the Northern Hemisphere. It includes a number of petrel groups, the relationships between which have finally been resolved to satisfaction. Shearwaters are medium-sized long-winged seabirds in the petrel family. They have a global marine distribution, but are most common in temperate and cold waters, and are pelagic outside the breeding season. These tubenose birds fly with stiff wings and use a "shearing" flight technique (flying very close to the water and seemingly cutting or "shearing" the tips of waves) to move across wave fronts with the minimum of active flight. This technique gives the group its English name.