NNZST/University of Auckland
Dr. Megan Friesen
Dr. Jingjing Zhang
Rako Buller's shearwaters (Ardenna (Puffinus) bulleri) breed only in northern New Zealand on the Poor Knights Islands. These islands are uninhabited, historically free of mammalian predators except for pigs on Aorangi (eradicated in the 1930s), and are strictly protected as nature reserves, managed by the Department of Conservation and Ngatiwai. Although breeding in New Zealand, this species spends their extensive non-breeding season foraging in the California Current off the coast of North America and on the Hawaiian-Emperor Seamount chain (an area of high fisheries activity).
During the Buller's shearwater breeding season (September to May) they are commonly seen in East Northland, Hauraki Gulf and Bay of Plenty waters, sometimes in large rafts of birds. It is partly due to their commonality and the large conglomerates that they form at sea, that this species has been considered to have a relatively large population size. The population was thought to be 2.5 million birds in the 1980s. More recent rapid surveys on Aorangi Island by Graeme Taylor suggested there may be fewer than 100,000 pairs using that island, less than half of the estimate made in 1981. Despite these speculations, a quantified population estimate had not been done for this species.
Evidence gathered by Graeme Taylor (2011-2013) suggested that foraging times during the incubation period of breeding Buller’s shearwaters may have increased in the past 40 years from 4 days up to 14 days. Additionally, Taylor’s tracking and new at-sea records of adults suggested they could be traveling further south in single foraging bouts during chick provisioning. These ecological changes, together with poor breeding seasons suggested that the population may be under stress. Our research has set out to address this concern.
Intensive field investigation included two parts to quantify populations and one part to quantify breeding success. This provided data for a population mode to establish an estimate of breeding pairs. Transects for burrow occupancy, random burrow density plots. For chick success 25m x 25m permanent plots were established to monitor chicks and breeding success. These burrows were studied at the beginning of the breeding season for activity and at the end of the breeding season for success (chicks ready to fledge), an important component in understanding the potential trends of the population and provides key baseline for future monitoring. Megan used Matt Rayner’s method to model population of Cook’s petrels on Hauturu, with ther team gathering habitat variables – i.e. aspect of slope, estimated canopy height, elevation, substrate depth, % of canopy cover and topographic features such as ridge, plateau, mid-slope, gully.
An article presenting the results of the survey will be published early in 2020. This will be the first comprehensive population survey for the species.
Diet & foraging studies
Seabirds are to some degree opportunistic feeders and they may, for example, be able accommodate changes within the marine environment by switching prey, or by feeding across different trophic levels, without impacting their reproductive output. The aim of this study is to fully investigate the diet and foraging distribution of Buller's shearwaters during breeding across three years, and the level of breeding success and corresponding chick development (growth rate and weight at fledging). While Buller's shearwater's foraging range during association with various types of fish schools. We are using GPS tracking to determine foraging breeding can be extensive, they also commonly observed foraging locally, mostly in ranges and behaviours during breeding. However, their diet is largely unknown. We are also collecting regurgitations and faecal samples (for DNA extraction and sequencing) and blood samples (for stable isotope analysis) to better understand the prey they are catching and the food they are feeding their chicks. With respect to birds returning from migration, blood samples collected will be used for both physiological and stable isotope analysis, the former to assess pre-breeding condition, their 'fitness to breed', with subsequent checking to see if they incubate and raise a chick, and if they fail, when they fail and why. Stable isotope analysis will allow comparisons of the trophic level at which the birds feed between non-breeding (northern hemisphere) and breeding (southern hemisphere) periods.