Huttons Shearwater Stocker Report

 Foraging behaviour of Hutton’s shearwater (Puffinushuttoni) off the Kaikoura Coast - Stocker Scholarship Report 

 

Della Bennet, Biological Sciences, University of Canterbury

 

The Hutton’s shearwater is an endemic and endangered alpine-breeding seabird that nests solely in the seaward Kaikoura mountains. In the mid 1960’s, eight colonies were located but by the 1980’s, only two sites remained, primarily due to pig predation. Until recently, research on the Hutton’s shearwater has been limited to monitoring population size, assessing levels of predation, and improving conservation efforts within the alpine colonies. 

 

In 2005, a fenced and predator-proof colony was established on the Kaikoura Peninsula to created a new population at a lowland site. However very little is known about the diet of Hutton’s shearwaters, their foraging behaviour while at sea, and how diet and foraging impact on breeding success. We are addressing these questions through stable isotope analysis (δ13C and δ15N) of Hutton’s shearwaters and their potential prey items, plankton and larval fish, along the Kaikoura coast.

 

Stable isotopes are forms of carbon and nitrogen found in an organism’s tissue and their composition and quantity can be used to pinpoint migratory movements and foraging locations of birds. During the 2014-2015 breeding season we collected feathers from chicks and adults within the alpine and peninsula breeding colonies. These feathers were then analysed to estimate the ratios of carbon to nitrogen, as differences in ratios are related to differences in the types of food ingested. Variation in C (δ13C from -14.79 to -19.40) and N (δ15N 8.09 to 17.43) was observed within individual feathers, between feather types and amongst individuals. This high level of variation suggested that Hutton’s shearwaters were feeding on a broad range of prey, although further comparisons of δ13C and δ15N ratios in both the birds and in the isotopic signatures obtained from their food are underway. 

 

To link the isotope ratios measured in the shearwaters with that of potential prey, we collected larval fish and plankton through tow sampling inthe sea off Kaikoura over a seven month period. These samples produced a ‘food web map’, which details the food chain from producers to consumers and predators. The results of the food web were then compared with the isotopic signatures from the feathers of the shearwaters. As with the feathers, the food web samples showed variability in isotopic signatures within fish species, zooplankton, and between months.  

 

In a few birds we were able to compare the isotope ratios from shearwater feathers which grew from late November to January,while birds were on the breeding grounds, and at the same time as the tows were collected. Somewhat surprisingly, the isotope ratios did not reflect the pattern expected if the birds had been foraging close to shore.This suggests that birds were foraging further offshore or were feeding on near-shore items not included in the food web.

 

To examine Hutton’s shearwater foraging behaviour in more detail, we also deployed time-depth loggers between November 2014 and January 2015 on breeding adults in the peninsula colony,monitoring a total of 45 dive days, and 19,570 dive events. The loggers are miniature devices  attached to the legs of adults which measure the depth that birds dive in the water.Amazingly, Hutton’s shearwaters were found to dive to an average depth of 4.7 m (maximum 35 m), and completed an average of 54 dives per day (maximum 348 dives). Diving depth and  frequency varied among individuals and time of day. We also found a difference in diving behaviour between birds incubating an egg and those feeding a nestling. 

 

Further work on the foraging behaviour of Hutton’s shearwaters is planned for the summer of 2016, but already our results have provided key data on the diving behaviour and diet of this species. Until now, conservation efforts have largely targeted terrestrial  environments used by the Hutton’s shearwaters, including predator control and restoration of breeding colonies. Knowing where birds forage and what they select as prey will help ensure the marine environments used by Hutton’s shearwaters are as equally protected.

 

Thanks to the North Canterbury branch of the Royal Forest and Bird Protection Society for their contribution towards this research project through the Stocker Scholarship.