Beating the odds
Almost two centuries after their ancestors were nearly exterminated, a small flock of flightless Campbell Island teal have been returned to re-colonise their home island deep in the Southern Ocean.
This teal, at a tiny 300–370 g, are the smallest, as well as the rarest, duck in the world and by a combination of lucky breaks and innovative rescue operations, has only narrowly escaped extinction.
Until the arrival of sealing gangs in the early 1800s Campbell Island was the undisturbed domain of millions of seabirds, hundreds of thousands of seals and sealions, and myriad small birds and insects. This unique living community was also home to a local species of teal, the smallest duck in the world. Its only natural predators were probably the opportunistic southern skuas, which will snatch unguarded seabird chicks, eggs and little ducks if they get the chance.
Over the next century the ecological balance of the remote island was disrupted by the introduction of sheep, goats and pigs which were deliberately liberated at various times to provide a food source for shipwreck survivors—a common practice among most seafaring nations of the time. In 1895 sheep were introduced again, this time for farming, and the island remained a leasehold sheep and cattle run for many years. When farming was finally abandoned in 1931, the livestock were left to run wild.
The cattle and sheep were eventually removed over several years with the last animals destroyed by the Department of Conservation in 1991. Cats, descendants of farm pets and probably at the extreme limit of their ability to survive in the harsh wet climate, appear to have died out at about the time the last of the sheep were removed. That left the accidentally introduced Norway rats, the toughest and most adaptable of the newcomers, still thriving on the island. They had found a paradise of small birds, large insects and an endless supply of seeds. The flightless teal had no defence against these tough new predators. Eggs, ducklings and even adult birds were all easy prey. By the early 1930s Campbell Island had a higher density of rats than any other measured landmass on earth at up to nine rats per hectare.
By the `30s, modification of the island ecology was widespread and the little teal had been almost wiped out on the main island. However, a small colony remained on Dent Island, a 26 hectare spike off the coast of Campbell Island. This population of between 30 and 50 birds was rediscovered in 1975 by a Wildlife Service expedition which found clear evidence of the grim situation faced by Campbell Island teal, and a plan was set up to rescue as many of the surviving birds as they could catch. Wildlife officers Chris Robertson, Rodney Russ and Gerry van Tets spent two hours on windswept Dent Island and managed to capture and study a single female before releasing it again.
In February 1984 waterfowl biologist Dr Murray Williams and Andrew Garrick were part of a Campbell Island expedition which revisited Dent Island to search for the tiny teal. They camped for two nights and of the very few birds seen they managed to capture a male teal and took it back to New Zealand.
Over the next six years other teal were captured from the island to form the basis of a captive rearing programme and by 1999 the teal population had been increased to 60 birds. All were held in captivity as there was no place safe enough to release them.
A year earlier, the Department of Conservation had undertaken a rat eradication project on Whenua Hou, a 1400 ha island five kilometres north-west of Stewart Island. The eradication technique was developed in New Zealand by innovative conservation officers using GPS-guided helicopters to drop poisoned cereal pellet baits designed specifically for rats. Many pundits suggested that Whenua Hou was too big for the system to be successful, but by 2000 the island was officially declared rat free and 24 teal were released there as an interim measure until their subantarctic home island could be cleared of predators.
By May 2001 Campbell Island had also been cleared of rats using the same system. At 11,300 ha, it is the largest island to be cleared of rats anywhere in the world.
In April this year a geological team on Campbell Island found a lone male teal, which had apparently swum the three km of open sea from Dent to the main island, and then another 20 km around the coast. Prior to the removal of the rats, this little voyager would have been snapped up as soon as he set foot on the island.
The Campbell Island pipit, which was also isolated on small rat-free islets, has self-reintroduced to the main island and is now rapidly re-colonising its former home range. Conservation officers hold hopes that the Campbell Island snipe, which was only discovered in 1997 on a single 19 ha islet by specialist bird recovery dog handler Dave Barker and his German short haired pointer Bob, will also make its own way to the main island.
In the meantime, the teal on Whenua Hou had held their own in the predator-free environment and, in early September this year, they were joined by a similar number of birds from a captive breeding population held at Mount Bruce in the Wairarapa. All 50 birds were returned to Campbell Island aboard HMNZS Canterbury—the same vessel which brought the first teal back to New Zealand in 1984—in September.
Unlike other endangered native birds on the New Zealand mainland, Campbell Island teal now no longer need human intervention on their home range, but a small population has been kept on mainland New Zealand at least until the fate of the birds on Campbell is known. They are now once more on their own and free to re-colonise the home range of their ancestors almost 200 years after the invasion which almost sent them into oblivion.