Our country's largest storm-petrel shares the genus's basic life-history strategy: nocturnal colony visitation, burrow-nesting, a single-egg clutch, slow chick growth. However, the concentration of so many storm-petrel species among only a few Baja and southern California islands is remarkable; their diversification seems related to body size and use of marine habitat. Of this group, the Black Storm-Petrel is the largest, is quasi-migratory, and is found the furthest inshore.
The Black Storm-Petrel is 9 inches long, with a wingspan of 22 inches, and weighs 2.1 ounces (59 grams). This large, dark storm-petrel has a dark rump, pointed wings, and a notched tail. Its flight is characterized by slow, deep wing beats and long glides. The bird's under parts are usually dark brown and the back is sooty black, with pale brown bars along the middle of the upper wings. Other storm-petrels that may be completely dark include the Leach's, Ashy, and Least; however, all are significantly smaller and only the Leach's flies in the same deliberate manner.
While this species breeds on at least six islands in southern and Baja California, the overwhelming majority (about 98%) breed on Mexico's Islas San Benito on the Pacific coast of Baja California. These islands probably host about a million breeding pairs (BNA). Other breeders are scattered about in small numbers on a few smaller islands in the Gulf of California and along the Pacific coast of Baja, north to the Channel Islands off of Southern California. This species vacates the breeding area during the winter and part of the population winters to the north to central California, while a larger portion winters south along the coast to northern South America. This split wintering distribution has developed in several seabird species in this region, perhaps as a strategy to avoid being trapped in the Gulf of California or along the Mexican coast during hurricanes.
Black Storm-Petrels spend most of their time at sea, visiting land only to breed. They frequent waters warmer than those frequented by the other large, all-dark Pacific storm-petrels, including tropical, subtropical, and warm-temperate waters of the extreme eastern North Pacific Ocean. Black Storm-Petrels nest in desert habitat on small rocky islands or the talus slopes of non-mountainous larger islands which are not large enough to sustain enough prey to satisfy predatory mammals year-round. In some cases, the birds have been relegated by predators to the precipitous portions of larger islands. Their dense cactus habitat on Baja California islands inhibits study of this bird.
While at sea, this species feeds on squid, small fish, and crustaceans that occur near the surface. In typical storm-petrel fashion, this bird often hovers just above the water with its feet hanging down, "pattering" on the surface and dipping to feed. Occasionally, this species will fly up off the water and plunge-dive to a depth of one meter to catch prey. This bird's large size may affect its prey choices. Specific prey includes larval spiny lobster, lanternfish, euphausiids, shrimp, and amphipods. These storm petrels also scavenge larger items of food by swimming alongside and pecking at them. They sometimes forage in the company of small, slow cetaceans like the bottlenose dolphin, which likely drive prey to the surface. The most active foraging occurs during crepuscular periods and nighttime. The birds are attracted by olfaction to fish oil slicks.
These birds favor rocky, offshore islands with sloping terrain containing clefts and cavities among and under rocks for nesting. As is true of most all-dark forms, Black Storm-Petrels do not excavate their own nesting cavities, but nest in burrows excavated by other storm-petrels and alcids. Practically all nesting islands in the Gulf of California as well as Islas San Benitos are shared with Least Storm-Petrels; in fact, both species may share nesting cavities, with the Black taking the larger chambers. Like most storm-petrels, the Black lays one egg per season and is only active around colonies at night. This nocturnal strategy allows them to feed and tend their nests during hours when Western Gulls, a predatory threat, are relatively inactive.
Individuals vacate breeding islands in the fall and return to the same islands and nesting burrows the next spring. Postbreeding movement occurs north, to waters off Baja California, Mexico, and central (Alta) California, and south, to waters off Central America and northern South America. Movement north precedes that to the south. Dispersal north and south from the center of the breeding range may be a strategy to avoid the numerous late summer and autumn hurricanes moving northeastward from tropical Pacific waters. Such fast-moving storms are dangerous to slow-flying seabirds, especially those trapped within the Gulf of California. This bifurcated migration is characteristic of many seabirds that nest along the coasts of Baja California.
Ainley, D.G., and Everett, W.T. 2001. Black Storm-Petrel (Oceanodromus melania). The Birds of North America, No. 577 (A. Poole and F. Gill, Eds.). The Academy of Natural Sciences, Philadelphia, PA, and The American Ornithologists' Union, Washington, D. C.
Cooper, D. S. California Important Bird Areas. Audubon California, Sacramento, CA. 2002.
