Foraging ecology of the Cook's petrel Pterodroma cookii during the austral breeding season: a comparison of its two populations

This study examined divergence in the foraging distribution, at-sea behaviour and pro- visioning strategies of a small procellarid, the Cook's petrel Pterodroma cookii, during chick-rearing at 2 islands off New Zealand, separated latitudinally by ~1000 km. There was little overlap in forag- ing distribution between adults from Little Barrier Island (LBI), which ranged to the west into the Northern Tasman Sea and east into the Pacific Ocean, and conspecifics from Codfish Island (CDF), which foraged west of the South Island in the south Tasman Sea in association with the subtropical convergence zone. Although birds from CDF ranged further than those from LBI, there was no differ- ence in mean foraging trip duration. Cook's petrels from CDF foraged over deeper, cooler water, with higher primary productivity, than conspecifics from LBI. At-sea behaviour also differed: adults from LBI spent less time in flight, and showed less variation in total flight time per day. Overall, Cook's petrels spend much more time in flight than albatrosses, and approximately the same amounts of time on the water during the night as during the day, suggesting a high portion of nocturnal foraging. Dive depths did not differ between colonies but were greater than expected for a gadfly petrel. Stable isotope signatures of blood indicated population-specific diets, and suggested that birds from LBI primarily consume cephalopods and fish, whereas those from CDF eat more crustaceans. Chicks at CDF received more food. These results suggest a broad divergence in foraging strategies between geographically well-separated colonies in response to regional differences in oceanography.