Population biology, predator prey dynamics, foraging ecology and conservation status of Pterodroma cookii

Abstract

The petrels, or Procellariiforme seabirds, are one of the most threatened amongst avian orders yet we Iack much critical biological data for the conservation of many species, particularly amongst smaller taxa. This thesis aimed to address shortfalls in ecological knowledge of the endangered Cook's petrel (Pterodroma cookii), breeding at two colonies in the north (Little Barrier Island) and south (Codfish Island) of New Zealand, and to use this species as a model for addressing methodological and theoretical shortfalls i∩ the conservation of small petrel species. Primary aims were to describe habitat use, to determine population sizes for assessment of conservation status, to investigate the synergistic impacts of varying introduced predator regimes on this species, and to begin to characterise its spatiai and foraging ecology. To facilitate modelling ofthe size ofthe Little Barrier Island Cook's petrel population, burrow use, distribution, and breeding habitat were described. The distribution of Cook's petrel burrows was disjunct with birds using unmodified high-altitude sites; anthropogenic disturbance was concluded as the primary cause of this habitat use. Relying on the habitat survey data, predictive modelling was applied to the census of the Little Barrier Island population, indicating approximately 286 000 pairs of Cook's petrel, six times that of previous estimates. Comparisons with traditional census methodologies demonstrated the usefulness of the predictive modelling approach. In addition, an island-wide survey was made of Codfish Island which revealed a population of approximately 5000 breeding pairs of Cook's petrel, recovering rapidly following predator eradications. Population estimates for Little Barrier Island and Codfish Island indicated a world population of 1 300 000 (900 000 - 1 800 000) Cook's petrels and downgrading of the conservation status of the species from endangered to vulnerable is discussed. To investigate synergistic interactions between petrels and their introduced predators, long-term breeding data for Cook's petrel on Little Barrier Island were used to test predictions of the mesopredator release hypothesis. In support of this hypothesis, cat eradication Ied to reduced breeding success of Cook's petrels through increased rat predation, though rat impact varied with elevation. As predicted, rat eradication resulted in an increase in petrel breeding success to a level higher than when cats and rats and rats had been present. These empirical findings suggest complex interactions between petrels, petrel life-history characteristics, habitat, and the impact of introduced predators. Finally, the feasibility of using light-weight geolocation loggers to track Cook's petrel was investigated with results indicating little impact on individuals' single foraging trips. Subsequent investigation of the foraging distributions and behaviours of Cook's petrel from Little Barrier Island and Codfish Island showed segregation of these populations in spatial patterning, foraging behaviour, diet, and chick provisioning rates, suggesting that the two populations should be managed under separate conservation strategies. In summary, this research has enhanced the conservation understanding of the Cook's petrel, and represents a broad perspective on the application of new analytical methods for the census of burrowing seabird populations. It has also contributed to our understanding of the ecosystem-level interactions between petrels and introduced predators in the context of different predator eradication regimes.