Summary
Three species of burrowing seabirds nest on the Diego Ramirez Islands (60 nautical miles south-west of Cape Horn): blue petrels (Halobaena caerulea), common diving petrels (Pelecanoides urinatrix) and sooty shearwater (Puffinus griseus). The islands are of high conservation value because they are one of the few remaining subantarctic island groups without introduced vertebrate pests (to which burrowing seabirds are highly vulnerable). They hold about 80% of the global population of blue petrels (Marchant and Higgins 1990, Warham 1990) and are the only known breeding site for this species in the south-east Pacific region. Since the discovery of the islands in 1619 the population size of burrowing seabirds has been estimated only once, in 1980 (Schlatter and Riveros 1997). To identify the current status and trends in the burrowing petrels, we estimated their population size in November 2002 using a novel technique: we applied a line-distance-sampling method to determine burrow density, and a burrow-scope to identify burrow contents. There were an estimated 1.3 million pairs of blue petrels (approximately half the 1980 estimate), 100,000 pairs of common diving petrels (2/3rds the previous estimate) and approximately 17,000 pairs of sooty shearwater (not recorded breeding previously) nesting on the islands in 2002. In this presentation, we will discuss the status of burrowing seabirds at the Diego Ramirez Islands and the utility of line-distance-sampling (as against area-search methods) for burrowing-seabird censuses.