Life through a burrowscope lens (Part 3) – subterranean Snares Islands

A Te Papa team recently visited the Snares Islands, 105 km south-southwest of Stewart Island, where they completed a range of seabird research projects. The most time-consuming task was a re-survey of the vast sooty shearwater (titi, or muttonbird) population there. Estimating the population size was based on two main parameters – the number of burrow entrances over the entire island (counted in plots and transects), and an estimate of burrow occupancy. One of the methods used to ascertain burrow contents is with a burrowscope – a small video camera on the end of a long tube. Here, Colin Miskelly (Curator Terrestrial Vertebrates) describes some of the subterranean inhabitants of the Snares Islands

Muttonbird burrows — Sooty shearwater burrows under *Olearia* forest on North East Island, Snares Islands Nature Reserve. Image: Colin Miskelly, Te Papa

Sooty shearwaters dominate the Snares Islands’ bird fauna in terms of sheer numbers and biomass. The numbers of birds that arrive each evening are staggering, as they arise from rafts offshore and wheel over the canopy. Peak laying is in late November, and so the birds we saw were a mix of those returning to lay or take the first incubation shift, and pre-breeders searching for a burrow or mate. While the smaller petrel species wait until it is fully dark before coming ashore (to avoid predatory skuas), many sooty shearwaters land before dark and start their incessant caterwauling.

Muttonbirds landing — Sooty shearwaters searching for a site to land at dusk, Snares Islands, December 2013. Note the vast numbers of birds in the background. Image: Colin Miskelly, Te Papa

Muttonbirds on forest floor — Courting sooty shearwaters on the forest floor at dusk, Snares Islands, December 2013. This image was taken in natural light at about 8 pm. Image: Colin Miskelly, Te Papa

The enormous size of the sooty shearwater population (1-2 million pairs), along with their large body size and aggressive nature means that the smaller species of burrowing petrels that breed on the Snares are excluded from most of the prime deep soil sites.

Muttonbird through burrowscope — A sooty shearwater on its nest, viewed through a burrowscope, Snares Islands, December 2013. Image: Colin Miskelly, Te Papa

The second-largest burrowing petrel species on the Snares Islands is the mottled petrel (korure). Most nest under tussock grass in thin soils near the cliff edge. We saw few during our visit, as most were still at sea, with the majority likely to lay in the week or so after we departed.

Mottled petrel through burrowscope — A mottled petrel incubating a freshly-laid egg, viewed through a burrowscope, Snares Islands, December 2013. The visible egg is of a fairy prion, which had been pushed off its nest by the larger mottled petrel. Image: Colin Miskelly, Te Papa

After the sooty shearwater, the most abundant bird on the Snares Islands is the much smaller common diving petrel (kuaka) – but their population would be almost impossible to survey in any detail. While many have discrete burrows on steep banks and in muddy areas, a large proportion of the population nest in small side chambers inside sooty shearwater burrows, entering through the same large entrance as the much larger shearwaters.

Kuaka through burrowscope — A diving petrel on its nest, viewed through a burrowscope, Snares Islands, December 2013. Image: Colin Miskelly, Te Papa

Two species of prion nest on North East Island (the largest island in the group, and where we were based). While both species are often found in soil burrows elsewhere, most prions on the Snares Islands nest in rock tumbles and caves.

Fairy prion through burrowscope — A fairy prion on its nest, viewed through a burrowscope, Snares Islands, December 2013. Image: Colin Miskelly, Te Papa

Fairy prions (titiwainui) remain as abundant on the Snares Islands as they were in the 1980s, but broad-billed prions (parara) are now much scarcer. This was the main species affected by the huge storm of July 2011, which was estimated to have killed over 200,000 broad-billed prions, plus lesser numbers of the five other prion species (see Te Papa blog links below). We had to work hard to find the ten broad-billed prion chicks we were seeking for collection of blood samples, to allow genetic comparison with the storm-wrecked birds.