There are four of us in the Murchison Mountains, walking slowly uphill through a boulderfield smothered in alpine flowers and shrubs. I am an all-too-infrequent visitor to such places, and my eyes are feasting on the garden at my feet: the glossy green saucers of Mt Cook buttercups, cushion plants that seem to glue the rocks together, the feathery tops of wild carrot and zipper-like leaves of whipcord hebe, spaniard with its wicked sock-piercing spines, the snow-white loveliness of gentian.

My companions’ focus is elsewhere. They scan the valley slopes, straining their eyes against the harsh sunlight, trying to coax an extra pixel or two of acuity that will enable them to spot the flicker of movement that signals a rock wren, New Zealand’s avian mountaineer. Every now and then they stop to twitch a bird caller, a cunning device that sends out a squeak when you twist its metal core.

We reach an old rockfall, a favoured wren habitat, and, sure enough, we spot a pair of wrens bobbing and curtseying and making short darting flights from boulder to boulder. After some discussion about the best place to erect a mist net (which works best if fitted into a natural depression in the landscape, so that the birds can be funnelled into it) we set up three aluminium poles and string a fine black mesh between them, reaching about three metres off the ground.

The net is constructed as a series of billowing pockets, so that a trapped bird will fall into the belly of a fold rather than bounce off a taut wall of netting. A pair of loudspeakers is set up, and a staticky recording of high-pitched tweets hisses out into the silent landscape. The stage is set.

Then we set about herding wrens, as if it were a sheepdog trial. Megan Willans, the Department of Conservation’s point person on rock wrens in Fiordland, acts as the heading dog on one side, with me on the other. We are helped by her colleagues; Martin Genet, the huntaway, works the target down the slope from above, while Hannah Edmonds crouches below the net, ready to disentangle the captive, should we be so lucky.

We try to predict the wren’s net-avoidance strategy while clambering down the rocks, shepherding it towards the net. Suddenly the bird makes a long, low flight into the meshes.

Edmonds disentangles the wren and hands it to Willans for banding while Genet organises “lunch”—half a grape, a dish of meal worms and a square of moistened bread—and puts the food into a transfer box. As Willans bands the bird, she points out a long loop of dead skin around one of its legs, like some sort of ethnic ankle bracelet, or a stocking that has fallen down. It is one of the primitive features of New Zealand wrens that they slough skin like a reptile.

Once our wren’s gaudy identification bands are in place, it is popped into the box, where it scuffles about briefly, then settles. We give each other high-fives, as if we’d just won a point in beach volleyball. But of course this is no game. We’re here because rock wrens are in trouble. Even here in the mountains, far from the nodes of human settlement, the rising tide of introduced predators is nipping at their toes. Mice enter their nests and eat the eggs. Stoats, stealthy agents of death, take adults and young alike. The birds are already living above the treeline, often nesting in almost vertical bluffs, where they can’t go much higher. In response, DOC has decided to establish a satellite population on a predator-free island—as insurance, in case the little mountaineers perish.