Abstract

The Atlantic Petrel, Pterodroma incerta, endemic to the Tristan da Cunha group in the South Atlantic and globally threatened, is one of the least known of the world's seabirds. At Gough Island, Atlantic Petrels are winter breeders, with single eggs laid in June–July and chicks fledging in December. Birds were exclusively nocturnal on land. The pre-egg exodus of ~50 days and nestling period of almost 140 days are near the longest recorded for Pterodroma species, and support the general pattern of prolonged development found in winter-breeding Procellariiformes. Chicks grew at a very slow rate, suggesting poor at-sea feeding conditions. Breeding success averaged only 20% and introduced House Mice, Mus musculus, were probably responsible for considerable losses of chicks in two breeding seasons. Preliminary population modelling predicts a decline in numbers. The potential impact of introduced mice suggests that the species' conservation status be upgraded from Vulnerable to Endangered and that further research on this issue is a high priority. The first quantitative estimate of breeding numbers indicates a population of ~1.8 million pairs of Atlantic Petrels. Provisional population estimates of Great Shearwaters, Puffinus gravis (980 000 pairs), Soft-plumaged Petrels, Pterodroma mollis (400 000 pairs), and Broad-billed Prions, Pachyptila vittata (1 750 000 pairs) confirm the international significance of Gough Island as a site for breeding seabirds.