Distress call: part two

Stalked by an invisible enemy (9 minute read)

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I narrow my eyes, peering through wire mesh, trying to spot Harry and Meghan. They aren’t easy to see, but after a bit of looking around, I catch a glimpse of movement and a flash of orange. There, camouflaged against smooth river stones, is a small bird coloured white, brown and black with an orange and black beak. It’s Harry, the male of the pair of shore plovers. He’s standing still, but every now and again he bobs his head. It’s the bobbing movement that has given him away.

Male shore plover (photo credit: Mireille Hicks)

I ask Mireille Hicks, who has looked after the shore plovers at Pūkaha for the last six years, about the head-bobbing behaviour.

“It helps with their depth perception,” says Hicks.

I’m intrigued by the idea that a bird needs movement to judge distance, but when I read about it, it makes perfect sense. Like most birds, shore plovers have their eyes on the sides of their heads. It gives them a wide field of vision, and helps them to spot approaching predators. But it means that they can’t look at an object with both eyes at once, which is how we humans judge distance. Instead, birds such as the shore plover use motion parallax. That’s the apparent difference in the speed that objects seem to move when we are moving ourselves – closer objects appear to move fast, while distant objects appear to move more slowly. To use motion parallax, though, the plovers need to be moving, so, when they are still, they bob their heads occasionally.

“Not very helpful for avoiding predators,” I say. “It makes them much more obvious.”

Hicks agrees with me, and goes on to tell me that this is even a problem in captivity. I’m surprised, because if there’s anywhere that you’d think the birds were safe, it would be in their aviaries at Pūkaha.

There’s a falcon that’s been bothering them here, she tells me. It can’t get them, but it swoops down, and the plovers get a fright and injure themselves against the mesh of the aviary. That’s why Harry and Meghan are alone in the main aviary – the other birds that were in there have been moved to other aviaries, with more shelter but smaller, until the falcon problem can be solved. How long that will be is hard to say. If the falcon could be caught, then it could be moved somewhere else, far enough away that it wouldn’t fly back. But, so far, it’s proved elusive. A better solution might be to modify the aviaries – adding a layer of shade cloth below the mesh so that the birds don’t hurt themselves when they fly into it. But the aviaries were built decades ago, and are now too fragile to cope with the necessary modifications. They really need to be replaced, but, as is often the case in conservation, the problem comes back to money.

For the shore plover, the falcon at Pūkaha is a further setback in a story that seems to be one setback after another. As I wrote in last week’s article, the shore plover was driven to extinction by introduced predators across most of its former range, until it survived only on Rangatira island in the Chathams. Attempts to re-introduce it to island sanctuaries close to mainland New Zealand have struggled, as the islands turned out to be not as safe from predators as hoped. And the numbers of shore plover are so low that the species can’t even cope with the native predators that it evolved with, such as falcons and black-backed gulls.

But there’s another problem facing the shore plover at Pūkaha, and it’s a much more insidioius enemy. Shore plovers, Hicks tells me, are among the bird species in New Zealand to be affected by a disease called avian pox. This surprises me. I’m interested in diseases (as you’ll probably have noticed from my writing), but I’ve never heard of avian pox before, or realised that native birds could be affected.

Avian pox is caused by a group of viruses related to the virus that once caused smallpox in humans. Like smallpox, avian pox viruses cause sores on the skin. Depending on the specific virus strain and the bird species infected, it can cause anything from a mild, even symptomless, infection through to lethal disease. Despite the name, it’s no relative of chickenpox, which is neither a pox virus nor anything to do with birds.

Avian pox viruses are found worldwide, but probably weren't in New Zealand before the arrival of Europeans, or rather European songbirds and poultry. Avian pox was first reported in a native bird, the pipit, in the 1950s, and then in silvereyes in the 1970s. However, it didn’t come up as a conservation concern until it was found in black robins on remote Rangatira Island in the 1980s. It was also found on some other birds there – tomtits, Chatham Islands warblers, introduced dunnocks and prions, a type of seabird.

The presence of avian pox was a blow to the already tiny black robin population. In one particularly bad year, when there was a high prevalence of avian pox as well as severe winter storms, the total number of birds declined from 35 to 25. Although the numbers have since recovered, it’s a reminder of the vulnerability of these small populations of rare birds, especially those as inbred as the black robin.

Since the 1980s, there have been increasing reports of avian pox among New Zealand’s native birds. So far, species reported to be infected include silvereye, robin, tīeke (saddleback), North Island brown kiwi, kererū, oystercatcher, pipit, weka and paradise shelduck, as well as the shore plover. Avian pox also infects common introduced birds, such as the blackbird, and it’s from the blackbirds that Pūkaha’s shore plovers are thought to pick up the disease. It’s carried by mosquitoes, and all it takes is for a mosquito to bite an infected blackbird, and then to bite a shore plover, and the shore plover will get sick. Once one shore plover gets the virus, it can pass it on to others, through direct contact, through food bowls and even in the ground of the aviary. That’s a real problem, Hicks tells me, because the virus can survive in an aviary for months, and there’s no obvious way to sterilise aviaries that have been built to resemble natural habitats.

Avian pox is a problem among captive-reared shore plover, but the presence of the disease on Rangatira Island raises questions about the impact of the disease in the wild population as well. There’s no record of it being on shore plovers on Rangatira during the 1980s. However, since we know they are susceptible, it seems likely that some shore plovers were infected.

There’s also a question about whether avian pox contributed to the extinction of the shore plover on the New Zealand mainland, as well as the historical decline and extinction of other native birds. Unfortunately, that may remain a mystery. The introductions of the blackbird and thrush, which both carry the disease, occurred in the 1860s and 1870s, just before the introduction of mustelids such as stoats. We know that mustelids had, and continue to have, a devastating impact on birds such as the shore plover. If avian pox did play a role in the decline of native birds, it would have been hard to distinguish that from the impact of mustelids and other predators.

Today, the shore plover has to live with the threat of avian pox. At Pūkaha, that means rigorous cleaning of items such as food bowls and, where possible, preventing the birds being bitten by mosquitoes. Some of Pūkaha’s aviaries have been fitted with a mosquito-proof mesh to try and prevent the infection moving from wild blackbirds to the captive shore plovers. The mosquito-proof mesh helps with the falcon problem as well, Hicks explains, because it stops the falcon from seeing the shore plovers in those aviaries. But it’s not possible to make all the aviaries mosquito-proof. In the end, some of the shore plovers are going to get avian pox, and it’s very difficult to do much about it. Fortunately, birds that survive avian pox are usually immune to the disease in future, and most of the shore plovers, at least the adults, do survive. Avian pox isn’t going to be the end of the species, but does make captive breeding and moving birds around more difficult. It’s another burden on an already endangered bird.

Even without avian pox and falcons, the shore plover is a difficult and delicate bird to keep in captivity. The birds are easily disturbed, by people as well as falcons. They prefer to nest under the cover of grasses and driftwood, which makes it hard to tell what they are up to. And sometimes, Hick tells me, you just have bad luck. One bird at Pūkaha died recently because it tried to eat a bee.

Inbreeding is also a concern. Most of the birds in captivity are descended from just 13 founder birds. Hicks gets advice from genetics researchers at Canterbury University before matching up birds for breeding, to ensure that pairs aren’t too closely related. Because of concerns about the gene pool, this year six new birds have been brought over from Rangatira for the captive breeding programme. It’s some good news in what has otherwise been a stressful time.

Breeding a delicate bird like the shore plover in captivity is difficult, but it’s what Pūkaha does. Over the last decade, 92 birds bred at Pūkaha have been released back into the wild. The real problem is that many of those haven’t survived, or have had to be brought back into captivity when their island refuges turned out to be unsafe. Right now, the shore plover is on Rangatira Island, Mangere Island (also in the Chathams) and Waikawa Island. Mana Island doesn’t look like a suitable location, after last year’s attempt to re-establish a population there failed. So getting the last of the stoats off Motutapu Island as soon as possible is critical.

Thinking about the survival of the shore plover, I remember something I read about the black robin. In 1992, when the black robin population numbered 138, Don Merton wrote that incredible good fortune contributed to the success of the Wildlife Service bringing the species back from the brink of extinction. There were so many events that could have taken a different direction. The plight of the black robin might not have been noticed in time. The birds might not have survived the difficult transfer from Little Mangere to Mangere Island. Old Blue, the ancestor of all today’s black robins might have lived a normal black robin lifespan of 5-6 years, instead of surviving – and breeding – until the age of 13. The Chatham Island warblers and tomtits might have rejected the black robin eggs that Merton and his team sneaked into their nests.

The black robin came near to extinction in a tiny fragment of forest on Little Mangere Island. However, once the Wildlife Service discovered their plight, their luck turned, and they can now be counted as a most fortunate bird. I can’t help thinking that the shore plover could do with a little bit of black robin luck.

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