In the latest audio survey of kiwi bird populations on New Zealand's North Island, many areas that were silent in 2016 now have airways filled to bursting with kiwi calls.

The male's high shriek contrasts with the females low growl, but regardless of which sound it was-the early morning hours in December, when the last survey was made, were filled with stifled cries of joy from 150 volunteer bird conservationists.

The manual population survey, called the Kiwi Call Count, uses the human ear to record population numbers of the nation's five kiwi species to assess the status of the birds in a given area. The same sites are used every year, and 2021 saw a 50% increase in the number of sites in which calls were heard, while not one site had become silent over the last year.

A complex kiwi-saving action plan to combat invasive species was implemented to protect the national bird in the early '90s. Stoats, dogs, feral cats, and other introduced mammals have flourished on the easily caught kiwi chicks, a species that evolved with no native mammalian predators.

"To sit out there and hear how many kiwi there are and how close they are-it makes the effort put into trapping worthwhile," Ayla Wiles, a biodiversity ranger for the Department of Conservation, told The Guardian. 

This trapping has been hugely successful, and in 2017 two species of kiwi-the northern brown and the rowi, waddled off the Endangered designation on the IUCN's Red List. Wiles told reporters that some areas are seeing more of a sprint than a waddle. A place called Whangerei Heads has gone from having 80 kiwi to more than 1,000 since the program began.

As a nocturnal bird, the call of the kiwi is the most reliable way to track the animals. The calls are easy to discern in the darkness, and even have unique characteristics that allow conservationists to recognize the same birds year after year.

"You can be trapping, you know, week after week after week, in the hope that your kiwi are doing well," said Ngaire Sullivan, an organizer at Kiwi Coast. "And then for four nights a year, for just those eight hours… You sit there and get to hear your outcome."

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