2011 was a terrible year for New Zealand seabirds. The Rena oil spill in October received the most media coverage and provided dramatic images (see Rena oil spill blogs). More insidious were the impacts of the Japanese earthquake and ensuing tsunami in March. A plume of radioactive fallout from the crippled Fukushima Daiichi nuclear power plant contaminated the North Pacific feeding grounds of several New Zealand-breeding species, including flesh-footed shearwater, sooty shearwater (muttonbird), Buller’s shearwater and mottled petrel.
However, the most extreme mortality event for any single species was a severe July storm estimated to have killed several hundred thousand broad-billed prions (see Riders of the storm – thousands of seabirds perish on New Zealand shores). The broad-billed prion is a poorly studied species. There are no well-monitored populations, and so there are few opportunities to determine the impact of the storm at a population level.
In the New Zealand region, broad-billed prions breed on small islands in the Chatham Islands, Fiordland, and around Stewart Island, and on the Snares Islands. A few breed on islets and stacks off Whenua Hou/Codfish Island, north-west of Stewart Island. I visited tiny Trig Island (a known broad-billed prion breeding site) there on 8 December, and found a ghost town.
The peaty soil was honeycombed with an estimated 500 burrows, but 90% looked like they had not been visited at all this season. In early December, most burrows should contain large chicks, but I found only four in about 50 active burrows inspected, and estimated only about ten chicks to be present on the island (i.e. about 2% of burrows contained chicks).
The storm that killed so many broad-billed prions hit 6 weeks before broad-billed prions begin to lay. The large number of active burrows on Trig Island that did not contain chicks may be due to an imbalance in the sex ratio of birds that survived the storm, or those that lost a mate may not have had time to find a new one before it was time to breed.
Feather samples taken from the four chicks handled, along with older skeletal remains found on the island, will be used as part of a Te Papa genetic study seeking to determine where the vast numbers of birds killed in July came from. Efforts will be made to collect genetic samples from other breeding sites as part of this study.
Additional information on the wildlife of Codfish Island can be found on https://blog.tepapa.govt.nz/2011/12/13/codfish-island-1934-and-2011-in-the-footsteps-of-edgar-stead-part-4/ and http://www.birdingnz.net/forum/viewtopic.php?f=3&t=1516
By Colin Miskelly, Curator Terrestrial Vertebrates