Introduction

The Townsend’s Shearwater (Puffinus auricularis townsendi) is confined to the Revillagigedo Islands off the western coast of Mexico, where its small population is jeopardized. Also threatened, the related Newell’s Shearwater (Puffinus auricularis newelli), once widespread in the main Hawaiian Islands, has been reduced to a few remnant breeding colonies. The taxonomy of these, and other members of the complex referred to as “Manx-type shearwaters,” is unsettled and has been frequently changed. As of 1983, the American Ornithologists’ Union considered Townsend’s a full species and reduced Newell’s to subspecific status at that time. Both forms are closely related to the Manx Shearwater (Puffinus puffinus puffinus) of the Atlantic. These shearwaters can be distinguished in the hand by comparison of morphology and plumage, but only the practiced eye can distinguish them on the wing at sea.

Habitat preferences and distributions of the Newell’s and Townsend’s shearwaters differ markedly. When not at breeding colonies, Newell’s Shearwater is highly pelagic, frequenting tropical and subtropical waters overlying depths much greater than 2,000 m, mostly to the east and south of the Hawaiian Islands.

In contrast, Townsend’s concentrates in cooler waters overlying the continental slope (isobaths of 200–2,000 m) of western Mexico and Central America. Both birds capture prey by pursuit-plunging, an uncommon foraging method among warm-water seabirds. Their flight is strong, with rapid wing-beats and little gliding, a style requiring predictable prey availability; thus, it is also uncommon among warm-water seabirds. These shearwaters rely heavily on tuna (Thunnus spp.) and other large, predatory fish that drive prey to near the surface.

The breeding phenology of these two subspecies does not overlap; Townsend’s chicks fledge in late spring, Newell’s chicks in late autumn. Other aspects of their breeding biology are little known, or at least much less known than for other Puffinus species. Predation from introduced mammals, one of the major factors affecting population trends of these birds, has confined nesting to extremely rough, heavily vegetated, and steep terrain, where access by humans is difficult at best. Pairs produce their single egg in a chamber at the end of a deep burrow. The incidence of nonbreeding among adults is high relative to that of other shearwaters. The chick-rearing period in Newell’s (unknown in Townsend’s) is longer than in the Manx Shearwater, owing perhaps to the low caloric content of the diet, largely squid, fed to Newell’s chicks (compared to the fish that the Manx feeds its young) or to greater commuting distances between colonies and foraging areas.

Habitat loss on oceanic islands—due to clearing of forests for agriculture and urban development, mining of cinder cones, and recent volcanic eruptions—is the major factor causing decline in the populations of these two forms. Urbanization and predation from introduced mammals are other significant factors responsible for decreasing populations in these birds.