Oceanodroma leucorhoa chapmani Berlepsch 1906

Oceanodroma leucorhoa chapmani Berlepsch

CHAPMAN’S STORM PETREL

PELAGIC DISTRIBUTION.—The pelagic distribution of this form is little known, and POBSP surveys have not been informative. Although the race is characterized by the fact that all members have a dark rump, its presence at sea is difficult to determine because it is indistinguishable from dark-rumped birds of both the Coronados and Guadalupe Island populations present in the same area. Although we have a considerable number of sightings of dark-rumped birds from both the northern cruise areas and the southern EASTROPAC areas, it is impossible to assign racial designation to these sightings. An admixture of collected specimens of dark-rumped, intermediate, and white-rumped birds of small size from the eastern Pacific are in most cases definitely assignable to socorroensis. It is not possible by measurements to separate definitely all dark-rumped socorroensis from chapmani. We have collected few, if any, chapmani at sea. On the basis of the limited number of small, dark-rumped specimens taken, however, it may be that this race winters much closer to the coast than do beali, leucorhoa, or socorroensis.

BREEDING BIOLOGY.—This dark-rumped form of Leach’s breeds predominantly or exclusively on the San Benitos Islands off west-central Baja California. During a POBSP survey of these islands from 26 to 29 April 1968, birds were plentiful on all three islands. At that time they were calling from the air, as well as occupying empty burrows and cavities among the rocks. No eggs were found. Several adults were discovered fatally impaled upon the spines of the rampant cholla cactus (Opuntia sp.). A later June survey noted 14 such individuals on one slope.

During a 25 to 27 June 1968 survey, most birds were still calling from empty nest sites; some 20 percent of the population were on fresh to heavily incubated eggs, but no chicks were found. The birds occupy every feasible habitat on the islands for nesting, with possibly the talus slopes and rocky areas preferred. In these habitats they are in competition for nest sites with the Black Storm Petrel and Least Storm Petrel (Halocyptena microsoma). Some chapmani used abandoned Cassin’s Auklet (Ptychoramphus aleutica) burrows, but a number were noted digging their own burrows. DeLong (POBSP) records that “despite the apparent weakness of the bony structure of the legs, they dig with great fervor and make the soil fairly fly.”

The species’ nocturnal and secretive nesting habits make it extremely difficult to arrive at more than a rough estimate of the population size. For the June 1968 survey this was set at 50,000 birds for the island group.