Breeding Biology of the Flesh-Footed Shearwater on Karewa Island

Abstract

Field trips were made to Karewa Island, Bay of Plenty, during the 1994/95 breeding season of the flesh-footed shearwater to coincide with estimated dates of laying, hatching and fledging. A sample of 94 burrows was selected on the first field trip. The chamber of each burrow was examined using a infra-red burrowscope on as many days as possible during each field trip. Most laying occurred between December 5-20. Incubation was estimated at 53-54 days, with most hatching taking place in the last week of January and the first week of February. Some chicks were close to fledging by April 30. From 80 study burrows, breeding success was estimated at 68.8% ± 13.1%, hatching success was 79.2% ± 11.5%, and chick survivorship was 86.8% ± 10.8%. From all study burrows, chick production (percent of burrows containing chicks) was estimated at 31.4% ± 8.3%. Five 5 x 5 metre quadrats were completed within the study area in 1995. Chick production in this sample was 20.0% ± 8.8%. The average length of burrows that fledged chicks was significantly different from the average length of burrows that were empty at the end of the breeding season. The size of Karewa Island was estimated at 5.23 ± 0.06 hectares. The size of the burrowed area was estimated at 2.01 ± 0.03 hectares, or 2.14 hectares corrected for slope. Using quadrat work completed by Department of Conservation staff in 1993 (Jansen 1993), the number of burrows on the island was estimated to be between 6,553 and 8,427 burrows (95% confidence limits). Using the conservative number of 6,553 burrows, between 1,514 and 2,602 chicks were produced on the island in 1994/95. Burrow density was estimated at 0.64 ± 0.17 m-2 using the five 5 x 5 metre quadrats. Mark-recapture work gave an estimate of 296 ± 90 diving petrels and 77 ± 16 grey-faced petrels within part of the study area. The consequences of people visiting Karewa Island, and possible methods for monitoring the colony were discussed. In response to the request from Tauranga Moana Maori Council to harvest chicks from the island, the population dynamics of the colony were modelled using the computer programme, VORTEX. Simulations showed that changes in adult mortality and immigration rates had the greatest effect on population sizes. Harvesting 600 chicks a year caused a 60% decrease in population size within 60 years, while a harvest of 150 chicks per year had little effect on the population size.