Black Stilt on the Brink
On stony riverbeds not far from Twizel, in the vast harsh landscape of the McKenzie Country, land that fries in summer and freezes in winter, the black stilt quietly clings to survival. Just 31 adults currently live in the wild, only one-third of them female. The black stilt, Himantopus novaezelandiae, was named kaki by Maori, after its yapping call. Once widespread throughout New Zealand, it was plentiful from the Bay of Plenty to Otago. By the end of the 19th century, they were disappearing from the North Island and by the middle of last century they were confined to the Mackenzie Country in the South Island high country. Today the black stilt is virtually restricted to the Mackenzie Basin, near Twizel township. Nearly all the remaining adult birds stay there year round, inhabiting braided riverbeds, lake deltas, ponds, swamps and tarns. The kaki has teetered on the verge of extinction for nearly three decades. In the early 1980s when it was discovered that only about 23 birds were left, the Wildlife Service began artificially incubating eggs and put in place a major predator-trapping programme. Over time this developed into the Black Stilt Recovery Programme, now run by the Department of Conservation (DOC). The programme’s aim is to recover the species to a level where its wild population is self-sustaining and needs no long-term management. Despite these efforts, the wild black stilt population has stayed firmly below 100; since 1987 the adult population has fluctuated between 40 and 60 birds. But hopes are now rising that recent management changes will boost numbers to the point where a self-sustaining population is once again a feature of the South Island’s braided rivers. Recovery Programme manager, Dave Murray, of DOC’s Twizel Area Office, says the potential for an increasing adult population is better than it has been for many years. “The total wild population, including all age classes, at the end of February 2000 is 81.” And there are a further 25 chicks waiting in the wings, reared in captivity from both wild and captive-laid eggs. The success is thanks to management changes that resulted in a quick boost to the population. Last summer, to avoid the loss of chicks before they fledged, the eggs of all wild birds were collected, incubated and the chicks reared in captivity, safely away from predators, floods and 4x4 vehicles. In the past, not all eggs collected were hatched in captivity. About half had been put back out with their wild parents just before the chicks emerged. Most died in the 40 days between hatching and fledging. This year’s change means double the normal numbers have survived to at least the fledgling stage. Dave says the ultimate measure of success will be for these birds to survive to maturity and enter the breeding programme. Two further changes, adding iodine to the diet of captive birds and providing supplementary food for a month after they are released into the wild, has increased the number of young birds surviving from 30 per cent to about 80 per cent. This, says Dave, has raised success rates from ‘very good’ to ‘exceptional’ by world standards. The changes came about following necropsy results on birds that had died soon after release – these showed they had goitre and were underweight. The iodine and supplementary feeding also means it may be possible to release young birds sooner, as three-month-old juveniles instead of nine-month-old sub-adults. An experimental release of juveniles has been carried out, with some success. If so, it means DOC’s incubator and brooding facilities could put through more eggs and chicks each breeding season, releasing the early birds in time to accommodate chicks from the second layings. The management changes were based in part on a 1996 technical audit of the Black Stilt Recovery Programme. In short, its findings were that black stilt management was doing the right things, but needed to do more of it. It recommended urgent research into the staggering lack of breeding success, finding more cost-effective ways to manage predators and to assess the effect of hybridisation. Hybridisation happens between kaki and New Zealand’s other stilt species, the black and white pied stilt, or poaka (Himantopus himantopus leucocephalus). Both probably originated in Australia. On a superficial level, the main difference between the two species is colour. As the kaki evolved, its plumage took on its distinctive black colour, its eyes became bright red, its legs grew shorter, its bill longer and stronger, and it became suited to living amongst shallow braided rivers, a feature typical of New Zealand but not Australia. But DNA analysis has confirmed that the kaki is a distinct species, evolving after arriving on our shores about one million years ago. The pied population probably flew in just 150 years ago, and has been spectacularly successful. In fact, so successful that today, with only about nine female black stilts left, the black stilt males have much more luck finding a pied stilt to mate with. The good news for the black stilt species is that hybrid females are less fit and their offspring seldom survive. The recovery programme hopes that, because kaki actively seek a black mate whenever possible, by increasing the wild kaki population it will also increase the percentage of black with black pairings and hybridisation will become less of an issue. And, says Dave, when necessary, mixed pairings can be discouraged. The current number of adult hybrids with dark plumage is 42. While hybridisation is a problem, breeding failure is considered to be the main cause of the kaki’s demise. A number of reasons exist - predation, loss of habitat, introduced weeds – and they can all be sheeted home to the arrival of Europeans in New Zealand. Black stilt had no defences against the predators which arrived on the coat tails of early European settlers. Ground predators were new to this ground nesting bird; until then attacks had been aerial – from Australasian harriers and New Zealand falcon. Today, their eggs and chicks are taken by rats, feral cats, ferrets, stoats, weasels, hedgehogs, Southern black-backed gulls and hawks. DOC has research underway to find out more about nesting failure, including round the clock surveillance of nesting pairs with chicks - stakeouts by day, and video monitoring by night. While the aim is to establish which predators are the most serious, the surveillance is also revealing other problems such as weak or sick chicks, chicks which do not feed properly, and disturbances by people, vehicles and stock. DOC has also been controlling predators at four nest sites during the past two breeding seasons to see what difference that makes to nesting success, and research is underway into what forms of predator control are the most efficient and cost effective. The results will come on stream in two years and lead to some further management improvements. The kaki’s continued survival is also threatened by changes in habitat. The birds particularly like banks of shingle or gravel in wide weed-free braided river channels. There they feed in the shallows, catching insects, molluscs and small fish, unusually large prey for a stilt. When Europeans arrived, braided rivers were channelled for flood protection, making their waters deep and steep and unfriendly for black stilt feeding. Wetlands were drained for farming so that today only remnants remain. Hydroelectric development has reduced water flows and riverbeds have been drowned under hydro lakes. And the Europeans brought weeds. Exotic grasses, Russell lupin, broom and willow have choked former feeding and breeding areas. These plants stabilise banks and build up large islands which force the river into deep fast flowing channels. And they provide extra cover for predators. Rivers upstream of the lakes in the Mackenzie Basin are still relatively natural, but those downstream are badly degraded. Because there isn’t much point breeding the birds if they have nowhere safe to live, Dave says DOC is doing everything it can to protect remaining habitats by influencing land use and protecting remaining natural areas. Since the early 1980s work has also gone into managing wetlands to provide them with good places to live and breed. Sometimes these are existing wetlands which have been recovered or modified, sometimes new ones have been built. Five wetlands are being managed, using weirs to allow the water level to be manipulated to create natural cycles and provide the best possible feeding and nesting habitat. Invertebrates soon moved into the newly established wetlands and released birds have made good use of the food source. Hopes are high that they will form pairs and nest there when they mature. Building a wetland can be expensive – one that now provides 10 hectares of water cost $50,000. Once the results of current research are known, in about three years, the department will know if further wetlands are needed. The department also makes every effort to let people visiting the area know how their behaviour, or their vehicles, disturb nesting birds. With the recent management changes the prognosis for the recovery of the kaki is good. Population modelling suggests that even a very small improvement in nesting success in the wild, about 10 per cent, may be all that is needed to ensure the long-term survival of kaki, without further captive management. The research currently underway to find out more about breeding failures and predator control will improve DOC’s ability to manage predators. While the objective is to establish a self-sustaining wild population or something as close to that as possible, if all else fails, Dave says it will be possible to maintain a wild population by captive releases alone. Kaki seem here to stay. © Marieke Hilhorst/ORIGIN NATURAL HISTORY MEDIA |