Site-specific reproductive failure and decline of a population of the endangered yellow-eyed penguin: a case for foraging habitat quality

Sub-populations within species can exhibit differing population growth trajectories in relation to one another depending on various environmental factors. In threatened species, negative population growth in some sub-populations can ultimately cause the demise of the species; therefore, understanding causal factors of population change is critical to inform management aimed at reversing population declines. Feral house cats Felis catus are potential predators of Endangered yellow-eyed penguins Megadyptes antipodes and were considered the principal causal factor in the species’ decline on Stewart Island/Rakiura, New Zealand. The number of yellow-eyed penguins breeding on Stewart Island is low relative to the number on close outlying islands, where cats are absent, and a census had recorded few juveniles on Stewart Island, suggesting poor reproductive success. Yellow-eyed penguin breeding attempts on the northern coast of Stewart Island and outlying islands were monitored for 5 yr, but predation by cats was not evident. Instead, disease, probably aggravated by starvation and poor dietary provisioning, was found to be a significant cause of chick mortality on Stewart Island. Reproductive success was consistently low there (0−33%), in contrast to outlying islands (27−76%). Little recruitment was recorded on Stewart Island, and the number of breeding pairs on the northern coast of Stewart Island declined by 27% between 1999 and 2008. Factors unique to the north coast of Stewart Island are believed to be adversely affecting nesting yellow-eyed penguins, as a similar decline was not recorded elsewhere on the island or on outlying islands.