Comparing the foraging locations and stress physiology of common diving petrels, Pelecanoides urinatrix urinatrix, between colonies in the Hauraki Gulf

Abstract

Within the Hauraki Gulf, common diving petrels (Pelecanoides urinatrix urinatrix), are an important species; forming breeding colonies on outer Gulf islands, and are tagged for extensive restoration to the inner Gulf islands. Using Tiritiri Matangi Island in the inner Gulf, and Burgess Island in the outer Gulf as study sites, the aim of this research was to assess whether inner Gulf breeding locations had any impact on the distance travelled to forage, as well as the overall health and stress of common diving petrels. This is especially relevant in this species given that they are better designed for underwater performance than long-distance flight, and yet must make daily foraging trips feed. As foraging may be especially problematic over the breeding season, when there is an egg to incubate or a chick to provision, we also aimed to investigate any changes in overall health and stress of common diving petrels across the breeding season. This was the first ever haematological study of this species comparing breeding stages or locations. GPS tracking of foraging locations found that Burgess Island common diving petrels foraged near their breeding grounds, travelling on average 46km per day, while Tiritiri Matangi birds travelled towards the outer Gulf, away from their nesting site, an average of 81km per day, and prey-switched to higher trophic level prey. They also significantly increased their average and maximum foraging speeds to achieve this in the same time as their Burgess Island counterparts. Comparisons of weight and stress measures between islands are inconclusive, but vary over the breeding season, indicating that the incubation stage is when birds are in the highest condition. Haematological factors indicate that the two populations adopt different strategies to increase blood oxygen capacity, but are inconclusive as to which population is in the best condition. Some but not all haematological variables changed across the breeding season. While our haematological and stress results were inconclusive, indicating that inner Gulf populations may not be experiencing a decline in condition because of increased breeding distances, this population may be at or near the end of its resilience to change, and thus may be more susceptible to adverse effects associated with climate change and other anthropogenic events on its environment.