Spatial variation in burrow entrance density of the sooty shearwater (Puffinus griseus)

Abstract

The effects of a range of habitat variables on spatial variation of breeding burrow density of sooty shearwaters, Puffinus griseus, were measured on 5 islands near Rakiura (Stewart Is) and 1 island in The Snares Is group, during the 2000-01 breeding season. Density estimates for 4 islands where Rakiura Maori harvest chicks ranged from 0.30 to 0.47 burrows per m2. Density on 2 non-harvested islands occurred at opposite ends of the burrow density spectrum (Whenua Hou, 0.09 entrances per m2; The Snares, 0.90 per m2). Burrow density was consistently lower in areas with shallow soil, in inland areas, and where there was more plant debris on the forest floor. The latter may reflect cause or effect because the birds drag woody and leafy debris into their burrows to form nests and to block the burrow entrance. Large amounts of variation in burrow density were not explained by habitat predictors. Detection of harvest impacts on sooty shearwater density on harvested and non-harvested islands will be more powerful if models account for soil depth and island edge-effects, but disregard vegetation variation.