Rangatira is no ordinary island. Unvisited except by scientists and conservation workers, it is nevertheless very densely populated indeed. Its 218 hectares of volcanic rock and soil offer refuge to as many birds as there are people living in all of New Zealand. And these birds nest in the soil in an area the size of central Wellington.
Rangatira (or, to give it its more prosaic Pakeha name, South East Island) is in the Chatham group, about 800 km east of Christchurch. Most of the year it can be a cold, unpleasant, windswept place, but for 3.5 million seabirds it is home during the one time of year that they are land-bound: during the breeding season. Several species of petrel can be found here-one rare and others in large numbers, such as the noisy sooty shearwater, the fairy-like white-faced storm petrel and the clownesque broad-billed prion.
Most of these birds lay their single egg and raise their chick in a burrow in a thin layer of soil which covers the island’s volcanic rock. So many of these burrows occur on the island that it is difficult to walk there without leaving a trail of destruction. Only by wearing “petrel boards”—snowshoe-like sheets of plywood attached to their boots—can conservation workers go about enhancing, rather than reducing, the survival chances of some of Rangatira’s endangered occupants.
The most critically endangered species on the island is the Chatham petrel, Pterodroma axillaris. Rangatira is the only place in the world where it still breeds. It was thought to have become extinct in the 1930s, but then, after more than 30 years with no records, a small population was discovered on Rangatira in the 1970s. The species’ continued existence has remained precarious ever since.
During most of the year, male and female Chatham petrels live independently somewhere in the Pacific, possibly flying to the northern hemisphere, but always on the wing and never coming to land until it is time to breed. Then, goaded by instincts that nobody really understands, they return for the breeding season (November-May) to this small speck of land.
They circle the island, uttering loud calls, then crash through the canopy of trees to the ground. Only metres away from the place where they were born, or bred in the previous year, they meet up with a new or previous partner. In what biologists call the “prospecting phase,” the pair finds itself a burrow- most likely the one it used the year before. After a brief courtship the birds mate and make a simple nest. Then, surprisingly, they part from each other for about a month-the “exodus phase.”
Well fed, both return to their burrow, where the female lays a single egg. Both birds then take turns incubating the egg, each for 10-14 days and nights in a row, for an average of 47 days. When the chick hatches, it is left alone in its burrow during the day while the adults go to sea to feed. The parent birds return at night to feed the chick. This pattern is followed for about two months, until the chick is heavier than an adult bird. The chick then comes out at night for a few weeks to exercise its wings-and lose weight.
Finally, no longer fed by its parents, the stronger, leaner and definitely hungrier chick climbs up a take-off tree and flies away-not to be seen again on Rangatira until its third year of life. The petrel’s natural life span is about 25 years. Birds start breeding at age 4 or 5, and are likely to breed each year until they die.
Death comes sooner rather than later for these birds, and usually via warm-blooded predators. In times of former abundance, fat petrel chicks in easy-to-reach burrows were a handy food source for the first human occupiers of these islands.
Well before Europeans arrived in the 19th century, the main populations of Chatham petrels were decimated on other Chatham Islands by mutton birders.
The introduction of rodents and other mammals probably hastened the birds’ fate elsewhere, but one hidden population on Rangatira remained-despite the loss of habitat that occurred even in this remote outpost.
As they did throughout New Zealand, European settlers cleared most of the forest on Rangatira to make way for sheep and cattle farming. Despite the scant, burrow-riddled soil—which must have resulted in broken legs and death for many animals—farming on Rangatira was profitable. The effect on the birds, however, must have been devastating, as every broken burrow during tile breeding season would have meant a broken egg, suffocated chick or starved fledgling.
Rangatira was purchased for a bird sanctuary in 1954, and a century of farming was discontinued in 1957. The cattle and sheep were moved off, and the last farm animal was shot in 1961.
Elsewhere in the trail of colonisation, mammalian predators followed in human footsteps, with devastating effects on species richness. This fate, however, has bypassed Rangatira: no rats appear to have lived on the island, not even during the farmed phase. As a result, insects thrive on the island—a fact which is especially evident at night, when a torch shone on the forest floor reveals a moving carpet of millions of arthropods. Most obvious among them are weta, several of which can be found per square metre or per tree trunk. Such abundance is what the New Zealand mainland forest floor must have been like before mammals arrived.
To make sure Rangatira stays rat-and mouse-free, bait stations are set all along the coastline, and no unauthorised landings are permitted.
A single pregnant female rodent could cause devastation to this fragile ecosystem, which includes not only the Chatham petrel but other rare birds such as the shore plover (also restricted to this island and one reef nearby), the famous black robin and Chatham Island species of tui, snipe, red-crowned parakeet, tomtit and warbler, for which Rangatira is the main sanctuary. The island is also a haven for various rare plants, insects and the Rangatira spider, one of the largest known in New Zealand.
The greatest enemy of the Chatham petrel is not a predator but a competitor, the broad-billed prion (Pachyptila vittata). Also a member of the petrel family, this species is abundant in the New Zealand region, but on Rangatira its numbers peak at about 660,000, and could still be increasing. That in itself wouldn’t be a problem, if only the prions didn’t nest in Chatham petrel burrows.
At the exact time of year when the petrel chicks have hatched and are waiting patiently to be fed, the prions come in to find suitable nest sites. They do this by exploring burrows which appear to be unused. A prion will put its head into a burrow, call out, and, if it hears no reply, may enter. If it finds a Chatham petrel chick inside, waiting quietly for its parents to return from a feeding trip at sea, it is then likely to kill the chick and claim the burrow for itself.
Given the very small number of Chatham petrels and the vast numbers of prions prospecting for burrows, this displacement behaviour is a serious problem. It is estimated that every petrel burrow receives inspections from several prions, which means that every Chatham petrel chick is in danger of being visited and either evicted or killed. Last year, one chick encountered as many as nine interferences from prions. Remarkably, it survived, due largely to the efforts of conservation workers.
For a number of years, the Department of Conservation has been trying to improve the breeding success of the Chatham petrel by locating active burrows and strengthening them against collapse, monitoring egg hatching and preventing excessive competition from the broad-billed prion.
When the adult petrels return to the island in November-December, known burrows are carefully monitored and protected by staff. When a bird is found in a burrow, it is marked with a red stripe of paint on its head. When its partner returns, it is marked with a white stripe. Most birds have been banded in a previous year, and their details are noted. Any unbanded birds found during the nightly searches are banded, and if no burrow is known for them a small radio transmitter is taped to their tail feathers. They can then be located in their burrows during daylight using a portable antenna. Only one in 10,000 burrow-nesting birds on Rangatira is a Chatham petrel, so the radio signal greatly improves the odds of finding the target birds.
Once a burrow is known to be in use, there’s a much simpler method than using a radio transmitter to monitor the comings and goings of its occupant. About the time of the birds’ return from their December exodus, every burrow entrance is fenced with three little sticks. Returning birds will hardly notice them and simply push them aside to enter their burrow. That event is recorded in the field notebook as “fence down,” after which a single check is made every other day to see if an egg has been laid.
Fences are restored daily to monitor the movements of the incubating birds, which are otherwise left undisturbed. Not until 40 days have passed are regular checks made to see if the eggs have hatched. Then the monitoring of the young begins, along with their nightly protection from prion incursions.
During incubation, disturbance of the nest is kept to a minimum to avoid spooking the adult birds. No more than once a week workers make sure the nest is still occupied—by a Chatham petrel, and not a prion. A simple test has been developed for this purpose. Stick your unprotected hand into the burrow. If it is pecked, there’s a bird inside. If the beak feels sharp and the peck painful, it’s a Chatham petrel. If the beak feels more blunt and spatula-shaped, and the peck more bearable, the occupant is probably a broad-billed prion.
Most of the petrel burrows are close together, in the central bushy part of the island, but visiting them all takes up to six hours a day. Although repeated walking along the same route has created tracks that are well compressed, getting to some of the burrows involves crossing very fragile areas. Petrel boards must be used—a slow and tiring way to travel.
Soon after the petrel chicks have hatched in February-March, DoC workers replace the natural burrows with artificial burrows which have strengthened walls and roofs made of wood and tunnels made of drainage pipe. The parents seem quite amenable to the changes to the burrows, so long as their young are safe inside.
At the same time as the petrel chicks are hatching, masses of prions start returning from sea to prospect for suitable burrows. To safeguard the chicks, conservation staff walk the tracks to check occupied petrel burrows two or three times a night, and once at daybreak. Any prion found inside a petrel burrow is removed.
Protecting the petrels is a time-consuming and labour-intensive operation—a desperate numbers game to redress the balance that was disturbed by human influence over the past century.
Is there any future in this laborious method of species preservation? This is a question that no doubt pops into the minds of weary conservation workers during their nocturnal rounds. DoC managers regard this regime as an extraordinary measure that is essential for the survival of the Chatham petrel. It is seen as a temporary solution, pending the development of other, less intensive methods.
Trials have been carried out to see if burrow entrances could be made less attractive to prions by covering the drainage pipe openings with neoprene covers which take extra effort to enter—just enough to discourage the prions, yet not so difficult to negotiate that the petrels, knowing their chick rests inside, are put off. A patented type of automatic door which opens when it senses a metal bird band is also being trialled. Meantime, nightly burrow checks remain the norm until the effectiveness of the new methods has been proved.
Why do the prions compete for petrel burrows in the first place? It seems that on such a small island there just isn’t enough space to accommodate everyone. And prions are not as picky as Chatham petrels are when it comes to site selection. They managed to maintain and possibly increase their numbers after the farming period, at the expense of the petrels, whose burrows were restricted to areas beneath the few remaining trees. Although much of the island is now reforested, Chatham petrels haven’t managed to recover to substantial numbers. And that recovery hasn’t been helped by prions killing petrel chicks when taking over their burrows.
It is worth noting that if there were sufficient space on the island and plenty of burrows to choose from, there would be little competition between Chatham petrels and prions. After all, the times of the year during which they breed on the island barely overlap. It is just too bad that the overlap happens at such a critical time, when the Chatham petrel chicks are so vulnerable.
Is all this effort worth the rescue of a single species in the long list of threatened birds? Why not let nature take its course and let the fittest survive? For Helen Gummer, who has spent two nine-month field seasons on Rangatira, the importance of the petrel preservation work is not in doubt. “It’s got to be done. We owe it to them,” she says.
And the efforts are paying off. In the absence of human intervention, prions are reported to have caused the mortality of 70-80 per cent of petrel chicks. But in 1999/2000, of 100 known eggs, 92 chicks hatched and 72 survived to fledging, thus becoming potential parents and producers of offspring in three years’ time. For 2000/01, of 114 eggs, 99 hatched and at least 86 (possibly 91) fledged.
One example illustrates how far DoC workers are prepared to go to ensure the survival of individual chicks. In 2000, one chick’s beak was damaged as a result of a fended-off prion attack. Without help, it was likely that the chick would develop an infection or a malformed beak, both of which would reduce its chances of survival. For two weeks, every morning the chick’s upper and lower bill were splinted together with a sterile strip of Band-Aid, and every evening the plaster was removed, so there would be no impediment to the parent coming in overnight to feed the chick. As chance would have it, the burrow was in a remote area. Survival of the chick was the only reward.
Conservation staff see this type of activity as a basic human responsibility. Never give up, they say, however desperate the situation. Hope is a strange phenomenon, and an incredibly strong motivator.