Structure of a Colony of Streaked Shearwaters on Mikura Island

Abstract

In 1985, a colony of streaked shearwaters (Calonectris leucomelas) with about 6, 600 burrows was surveyed in the Ohshimawake-gawa valley, on the north-eastern part of Mikura, an island in the Izu Island group off central Honshu, Japan. This colony, covering 3.4ha was located on a coastal cliff more than 300m high, and its burrow density was higher (over 0.5/sq.m) along the cliff edge than at areas away from the edge. Complication of the burrow structure by frequent use corresponded to the increase of burrow density. The degree of cover of shrubs and grasses decreased as the burrow density increased; more than 40% of the colony had less than 40% of the CD value. The cliff edge was the most frequently used site for take off, followed by inclined trees, 6-12m high, away from the edge. Few trees were used for take off on the lower slope near the valley bottom. Annually eliminated soils by burrowing were far greater in volume than natural soil formation from the materials, and the surface soil erosion in the colony was from several to some dozen mm per year, being several tens to hundreds times the volumes in natural areas with dense ground cover. The period of use of the colony was estimated to be 40-230 years using those erosion speed and the soil depth of 60±42(SD)cm(n=11).