Abstract
Pterodroma hypoleuca were observed in the Northwest Pacific (12°-46°N, 128°E-172°W) during several cruises on the R. V. Tansei Maru and R. V. Hakuho Maru between 1974 and 1983.
(1) Although a few P. hypoleuca remain in the vicinity of the breeding islands (Ogasawara, Volcano and western Hawaiian islands) and in the Northwest Pacific throughout the year, the majority move to northern areas such as the waters off Sanriku and east of Honshu in May and June for feeding. They are widely distributed in these areas until July and the beginning of August, but disappear from the northern areas by the end of August or the beginning of September. Birds returning to the breeding areas were found in the vicinity of the Izu Islands after October.
(2) P. hypoleuca were not observed in the area north of 40°N. It appears that the northward movement is influenced by surface water temperatures until July. After August northward movement ceases and birds begin to return to their breeding islands.
(3) During their migration between the breeding islands and the northern areas P. hypoleuca were found to be solitary. But flocks consisting of two or more birds were frequently found, feeding or resting, in northern areas. P. hypoleuca usually gather in a single species flock, however mixed flocks have been observed with P. externa cervicalis, P. nigripennis, Bulweria bulwerii and Puffnus pacificus.
(4) P. hypoleuca occurred in areas where surface water temperatures ranged from 16°C to 29°C; they were particularly abundant in areas of 18°C, between 20°C and 23°C, and between 26°C and 27°C.
