The austral summer is the peak breeding season for seabirds in New Zealand. Te Papa is undertaking research into the impacts of seabird mortality in fisheries on shearwater populations over the next 2 summers. We began the study in January 2012, surveying islands to assess changes in the numbers of breeding birds. We combined transect surveys (measuring density of birds nesting in burrows), with studies of the foraging ecology of two species of native shearwater, the flesh-footed shearwater and the sooty shearwater.
During the summer season the shearwaters are nesting in their burrows, they take turns sitting on the egg, one parent waiting patiently in the dark whilst the other is at sea feeding for around five days. Shearwaters fly huge distances, the Sooty Shearwater can cover up to 64,000 km annually and can fly at over 64 mph. Part of this work was to try and establish the exact movements of these birds during the nesting period by fitting them with tracking devices for several days. The loggers were taped under the feathers on the back of the bird and double secured with mini cable ties. The fixing had to be ultra secure as, incredibly, these birds can dive to depths of 60 m for their food – and we wanted the data back. This work is being done in collaboration with Associate Professor John Arnould, Deakin University, Australia.
Shearwaters are active at night, shortly after dusk the birds return to the island, this is signified by a sudden loud whump, followed by rustling as they make their way through the foliage and into their burrows. Recognising the shape of the trees and bushes from the air is how they locate their burrow, and then they drop through the tree canopy, land with a thud and walk the rest of the way home. They start the day with their version of the dawn chorus before heading out to sea to feed.
Titi Island has sheer cliffs to one side with barely any vegetation, the south side is steep sided and has gullies interspersed by stony ridges. The birds make burrows in the deep soil of the gullies, these are used for many many seasons, often by the same breeding pair. Our job was to estimate the size the seabird population and track the changes through time. Our results are being compared with a similar study undertaken in 2006 – 2011.
Finding the burrows on the steep slopes, hidden under the vegetation and fallen trees was the first task, even on the bare earth slopes, leaves and twigs could fill the indent which was all that indicated an entrance. We had to crawl around carefully and check out each possibility, the earth above each burrow was fragile and the roof could be very thin.
We also had to define the nesting areas within each gully as seabird populations tend to expand outwards from the existing colony and we wanted to establish what changes had occurred since the last survey. There were eight colonies on Titi and the perimeter of each had to be established with GPS reference points created at frequent intervals so the dots could be joined and the outline would be known, any future change in this would show population increase. We did this work on two other islands over the 2011/12 shearwater breeding season; Ohinau Island, off Whitianga in Coromandel, and Lady Alice Island in the Hen and Chickens group off Whangarei were our other survey sites.
Bycatch in fisheries is a pervasive problem for long-lived species such as shearwaters. Fishing deaths occur when birds get hooked on longline hooks, or entangled in trolling lines, trawl nets and setnets in both recreational and commercial fishing in New Zealand. The level of bycatch for some species is of concern nationally and internationally. Our research is helping to clarify whether species such as the flesh-footed shearwater are holding their own at breeding sites in spite of persistent deaths through bycatch.
Loggers deployed, recovered, and over 500 burrows inspected by burrowscope, we headed home after our exertions, enjoying a gourmet meal of mussels at Havelock – after much needed showers. This study is funded by the Conservation Services Programme of the Department of Conservation. Thanks to the team at DOC in Havelock and Nelson for assistance with the study, and much needed weather reports.