Anatomical studies have shown that a remarkable amount of morphological variation exists among shearwater species. In the genus Calonectris, the birds are lightly built, with long wings and have a weakly developed sternum, features well adapted to effortless gliding flight but poorly adapted to the use of the wings in swimming under water. Members of the genus Puffinus show a progressive trend towards a sturdy build, with short wings, well-developed sternum and pelvis, and streamlined, compressed tarsus. These features are well adapted for swimming under water with wings and feet, but they require a more laborious flapping rather than gliding flight when the bird is in the air. This morphological variation is related to differences in modes of feeding. The diving and underwater swimming of shearwaters [Puffinus griseus] off Washington, USA, is described and discussed.