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This is Scientific American — 60-Second Science. I'm Christopher Intagliata.

For more than a hundred years, this sound has been missing from New Zealand's forests: (hihi call up and under) "It's like two stones being clicked together, or two marbles being clicked together. It's a very simple call." Oliver Metcalf, a conservation biologist at Manchester Metropolitan¹ University, has studied the small bird that makes the noise—the hihi. By the 1880s, it was all but wiped out by rats and disease, which came along with colonists².

But the hihi did manage to survive on a predator-free barrier island called Te Hauturu-o-Toi. And now they're being reintroduced to mainland New Zealand. "And they're going through the process of trying to restore the ecosystems³ they had prior to colonial settlement there. So they're bringing back the birds one by one."

But the dense⁴ forests, and the birds' unusual behavior, makes them tough to monitor. "They're very inquisitive—they love people! So they'll come and see you. If they hear you coming, they'll come and see you. But that means you have a problem of knowing what they would have been doing when you're not there."

So instead, Metcalf and his collaborators from the Zoological Society of London u?sed that distinctive⁵ call (hihi call) to their advantage—using an array of audio recorders to eavesdrop⁶ on 40 hihi birds reintroduced to the North Island's Rotokare Scenic⁷ Reserve. A month later, analysis of the calls revealed that the birds had abandoned some areas of the reserve, and settled into others... suggesting they'd begun to form territories, often close to water.

The details are in the journal Methods in Ecology and Evolution.

Metcalf says the concept could be used to monitor all sorts of animals. "So whether that's a reintroduction of a small mammal, or how existing birds species respond to a housing development nearby or something like that, this process could be used to understand how those populations respond to changes."

Assuming, that is, they make some noise.

Thanks for listening for Scientific American — 60-Second Science. I'm Christopher Intagliata.