Predation at Sooty Shearwater Puffinus griseus colonies on the New Zealand mainland: is there safety in numbers?

Abstract

Burrow occupancy, survivorship, and breeding success were assessed at eight Sooty Shearwater Puffinus griseus colonies along the southeastern coast of New Zealand during the 1994/95 and 1995/96 breeding seasons. An average 46% and 47% of burrows contained breeding birds, but only three colonies had chicks survive to fledge. Most breeding failures were at the egg and early-chick phase. A large proportion of adults were killed at some places, causing the extinction of two of our study colonies. Predation by Stoats Mustefa erminea was the main cause of breeding failure and adult loss at most mainland colonies, but Norway Rats Rattus norvegicus were the principal predator at one colony. Protection of adults, eggs and young chicks from predators is crucial if mainland colonies are to persist. A conceptual model predicts that predation becomes part of the extinction vortex as the size of a colony dwindles because the depredations of a few rogue predators have catastrophic impacts on the few remaining birds.