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Black-vented Shearwater
Puffinus opisthomelas
Order
PROCELLARIIFORMES
– Family
PROCELLARIIDAE
Authors: Keitt, Bradford S., Bernie R. Tershy, and Donald A. Croll

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Introduction

Black-vented Shearwater adult, in flight.
Figure 1. Distribution of the Black-vented Shearwater.

This small shearwater, endemic to the Pacific Coast of North America, is named for the dark under tail-coverts that separate it from other closely related “white-vented shearwaters” in the genus Puffinus . Like most other Puffinus species, the Black-vented Shearwater is nocturnal when visiting land. When sandy substrate is not available for burrowing, this species lays its single white egg at the back of a natural rock crevice. Appearing deserted during the day, nesting colonies come alive with the moaning wail of shearwaters during moonless nights.

In contrast to its more pelagic relatives, the Black-vented Shearwater is a coastal species, most frequently observed within 25 km of shore. It is the only shearwater that breeds at sites within the California Current, nesting on remote desert islands near productive upwelling zones off the west coast of Baja California, Mexico. In feeding, this shearwater plunges from just above the sea surface or submerges from afloat and dives to depths >20 m to catch schooling fishes, squid, and probably crustaceans.

A paucity of information on the breeding biology and status of this bird led the International Union for the Conservation of Nature (IUCN) to list it as a Species of Concern and Mexico to list it as Threatened. Although recent information shows that populations are larger than previous estimates suggested, >95% of the breeding population occurs solely on Natividad I., Baja California Sur, Mexico. In addition, all breeding locales have historically had introduced mammals and permanent human residents, a situation that has reduced available nesting habitat and increased adult mortality.

The coastal foraging habits of this species and its erratic postbreeding dispersal stand in contrast with many other Puffinus shearwaters, making this an interesting species for study. Very little is known about the bird, owing primarily to its breeding on remote islands, and its nocturnal, burrow-nesting habits. Besides a collection of egg records from the 3 breeding islands, knowledge of reproduction is limited almost entirely to Natividad I. (Keitt 1998). However, postbreeding movements northward are well known from observations off California, and these movements have been linked to ocean temperatures (Ainley 1976, Briggs et al. 1987, Tyler et al. 1993, Ainley et al. 1995, Veit et al. 1996). Future research on foraging energetics and food provisioning of young of this coastal species should prove rewarding.