Sooty Shearwaters off California: Distribution, Abundance and Habitat Use

Sooty Shearwaters (Puffinus griseus) migrate to waters of the North Pacific Ocean each spring after nesting on colonies in the South Pacific. Their numbers and habitat affinities off California were investigated during 60 aerial and 24 ship surveys in 1975 to 1978 and 1980 to 1983. Population densities computed monthly indicated arrival throughout California waters in late March and departure of most birds from southern California one to two months earlier than in waters to the north. Peak densities exceeded 10 birds km-2 over the shelf and upper continental slope off central California for two to six months each year but reached these levels on only a few occasions in northern and southern California. Densities in all areas generally were highest in late spring and reached secondary peaks in mid- to late summer after variable declines from the spring peak. Populations are estimated to total 2.2 to 4.2 million birds in May, June, or July, depending on year and method of calculation. Population turnover may be rapid; total numbers of birds migrating through the area may be much higher than the "instantaneous" population. Shearwaters concentrated in relatively shallow, cool waters, especially where strong thermal gradients marked the edges of upwellings.