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While most shearwaters are encountered on the open ocean, Black-vented Shearwaters are frequently sighted from shore. This species nests on isolated islands in the waters off North America's Pacific coast. Seemingly deserted by day, these nesting colonies come alive at night. The small Black-vented Shearwater is named for the dark under-tail coverts that distinguish it from closely related "white-vented shearwaters." Its coastal foraging habits and erratic post-breeding dispersal are also different than that of many other shearwaters.
Our smallest Pacific shearwater, the Black-vented is 14 inches long, has a wingspan of 34 inches and weighs 9 ounces (270 grams). The Black-vented Shearwater is named for the dark feathers under its tail. This bird has dark-brown upper parts and is whitish underneath, including under its wings. Male and female birds are identical in plumage, with some birds showing white mottling along the neck, shoulders, and flanks. Similar-appearing species that overlap in range include Pink-footed, Townsend's, and Manx shearwaters.
During the non-breeding season, Black-vented Shearwaters can be found south along the Pacific coast of central Mexico and north to Monterey Bay in California, generally preferring to stay in waters above 14 degrees Celsius. This species is most common off southern California in December. Breeding is restricted to isolated islands off Baja California, Mexico. These include Natividad Island, San Benito Island, San Roque Island, and islets off of Guadalupe Island, all of which have been identified as Global-level Important Bird Areas. Natividad Island has been estimated to host as much as 95% of the global breeding population of this species.
Black-vented Shearwaters are most often seen in flight, though they may rest on the water when the wind is light. In winter, these birds inhabit coastal waters from Monterey, California, to central Mexico, generally preferring water temperatures greater than 14°C. The distribution and abundance of this species appear to be influenced by warm-water events. In contrast to its more pelagic relatives, the Black-vented Shearwater is a coastal species, most frequently observed within 25 kilometers of shore. It is the only shearwater to breed at sites within the California current, primarily near the productive upwelling zones off the west coast of Baja California, Mexico. Black-vented Shearwaters prefer to nest on remote desert islands over 200 meters in elevation, with little vegetation, and no native mammalian predators. On Natividad Island, the breeding habitat consists of stabilized dunes covered with introduced ice plant (
Mesembryanthemum crystallinum), which are suitable for digging burrows; on San Benito and Guadalupe Islands, the birds more commonly use rock crevices for nesting.
Black-vented Shearwaters feed primarily on small schooling bait fish such as anchovies and sardines, squid, and probably crustaceans. Prey is caught by pursuit-plunging (shallow dives from the air), dipping, seizing prey on the surface from a floating position, and diving underwater to depths of greater than 20 meters, using the wings to propel. The bird swallows its prey whole below the surface—likely one prey item per dive. These shearwaters have strong pincer-like bills, and their tongues and palates are lined with backwards-facing papillae (bumps), which are well adapted for catching and holding prey. A compressed tarsus and narrow pelvis promote efficient diving. Semi-permanent eddies north and south of Point Eugenia on the Baja California coast, may help concentrate prey for Black-vented Shearwaters. Black-vented Shearwaters may follow boats dispersing bait, though they do so less than other shearwaters.
Like other shearwaters, this species nests underground and only visits its colony at night; nesting colonies can seem deserted during the daylight hours. Burrows are excavated in sandy soil where possible, using the bill and feet. When sandy substrate is not available for burrowing, this species lays its single white egg at the back of a natural rock crevice. The species will also use artificial burrows. Once built, adults tend to return to the same nesting burrow year after year. Both sexes seem to share the responsibilities of caring for the egg and chick. On average, incubation lasts 49 days. Once hatched, parents feed the chick is fed partially digested regurgitated fish and other prey. The chick exits its burrow for several nights to exercise its wings prior to fledging at about 68 days of age. Fledglings are independent of their parents when they leave the nest.
No classic migratory behavior is known for this species. Black-vented Shearwaters are known to visit breeding colonies at least 10 months out of the year. Many individuals remain near their breeding areas year-round. Some disperse south, and more often, northward, after the completion of breeding in July or August, but the birds typically stay within 25 kilometers of shore.
Keitt, B.S., Tershy, B.R. & Croll, D.A. 2000. Black-vented Shearwater (
Puffinus opistomelas). In
The Birds of North America, No. 521 (A. Poole and F. Gill, Eds.). The Academy of Natural Sciences, Philadelphia, PA, and The American Ornithologists' Union, Washington, D. C.
Stattersfield, A.J., and D.R. Capper (Eds.).
Threatened Birds of the World. Lynx Editions and BirdLife International, Barcelona and Cambridge. 2000.
