Te Papa vertebrate curator Dr Colin Miskelly recently led a team that visited Takapourewa / Stephens Island Nature Reserve, to select and gather up 100 fairy prion chicks to move to Mana Island near Wellington. This is the sixth (and last) in a series of blogs about the project and the wildlife of Takapourewa.
As described in previous blogs in this series, a burrowscope is a high-tech tool used to view the contents of burrows, and is similar in design and function to an over-sized endoscope. The camera lens is on the end of a long tube, and is surrounded by small infra-red light sources, meaning that the image produced is in black-&-white. Burrowscopes are widely used in conservation research and management in New Zealand, particularly in studies of burrow-nesting seabirds. New Zealand has far more breeding species of petrels (including shearwaters and prions) than any other nation, and 33 of our 37 breeding species nest in burrows. All petrel species lay a single egg per breeding attempt.
The fairy prion is by far the most abundant petrel species on Takapourewa, with an estimated 1,830,000 pairs. This is the largest known colony of this species both in New Zealand and in the world. Their burrows are found at a density of about two per square metre over much of the island, and many tunnels connect up to form complex burrows with multiple entrances and multiple nest chambers. This subterranean world provides shelter and food for a wide range of other species. In addition to the burrows they provide, prions and other seabirds have a huge impact on the island’s ecology by delivering tons of marine-sourced nutrients in the form of droppings, spilt regurgitations, shed feathers, corpses and failed eggs.
We were searching for 120 fairy prion chicks that matched our pre-determined weight and wing-length criteria, from which we would eventually select 100 for translocation to Mana Island. This required checking 1655 prion burrows over three days, and the opportunity to encounter some of the other species that live below the ground on Takapourewa.
The best-known (and most studied) burrow-inhabitant on Takapourewa is the tuatara. The island holds by far the largest population of this medium-sized egg-laying reptile, which has no surviving near relatives outside New Zealand. While tuatara are capable of digging their own burrows, most of the estimated 30,000 to 50,000 tuatara on Takapourewa make use of burrows dug by prions. This is an apparently one-sided relationship, with no recognised benefits to the prions, while tuatara find both shelter and food in the prion burrows. In addition to the wide range of lizards and insects that live in the burrows, large tuatara occasionally prey on prion chicks.
Four other burrow-nesting seabird species breed on Takapourewa, and the three largest species (sooty shearwater, fluttering shearwater and little penguin) often take over and enlarge prion burrows. The sooty shearwater is the same species that breeds in vast numbers on islands around Stewart Island, where the chicks are harvested as muttonbirds in April and May. Their eggs hatch in mid-January, with some pairs caring for recently-hatched chicks at the time of our visit.
All petrel chicks are covered in dense down on hatching, and are able to maintain their own body-temperature within a few days, so their parents soon leave them “home alone” while both head out sea to find food. But the young chicks have few defences against predators, explaining why most species have been wiped out on the New Zealand mainland. Many petrel species share some of their island retreats with tuatara. Fortunately for the petrels, predation rates by tuatara are far lower than can be inflicted by rats, stoats, pigs and cats.
While searching for prion chicks, we also found geckos, skinks and giant weta sharing their burrows. Within the grassland, the main lizard species living in the prion burrows were raukawa geckos, spotted skinks and northern grass skinks, while under the forest, speckled skinks were often found in prion burrows. In addition to giant weta, many smaller insect species live in bird burrows on Takapourewa, including jumping weta, ground weta, darkling beetles and ground beetles, all of which were too small to show up clearly on the burrowscope screen.
Other blogs in this series