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On O'ahu, Wedge-tailed Shearwaters (Puffinus pacificus) and other seabirds nest primarily on small offshore islets, but fossil evidence shows that many seabirds formerly bred on O'ahu itself. Predation by introduced mammals is suspected to be the primary factor preventing shearwaters and other seabirds from reestablishing large nesting colonies on O'ahu. We investigated the ef- fects of predation on Wedge-tailed Shearwaters by comparing three small un- managed colonies at Malaekahana State Recreation Area on O'ahu, where feral cats are fed by the public, with a large managed colony at nearby Moku'auia Island State Seabird Sanctuary, where predators are absent. During three visits on 19 April, 16 June, and 23 October 2000, we located 69 occupied burrows in three colonies at Malaekahana and 85 occupied burrows in four monitoring plots at Moku'auia. Many more nests produced chicks at Moku'auia (62%) than at Malaekahana (20%). Among plots at Malaekahana, reproductive success was lowest (zero) at the colony closest to the cat feeding site. In addition, 44 adult shearwater carcasses were found at Malaekahana near the cat feeding site. Pre- dation, most likely by cats attracted to supplemental food, had a devastating impact on shearwaters at Malaekahana. At one colony there was complete re- productive failure and almost all adults were killed. Populations of long-lived species like seabirds are sensitive to adult mortality, and Malaekahana may act as a sink, draining birds away from other areas.

Comparison of Managed and Unmanaged Wedge-Tailed Shearwater Colonies on O'ahu: Effects of Predation