Saving the rare hihi bird by hearing its happy call

From the native New Zealand hihi bird’s point of view, translocation must be akin to being abducted by aliens, muses Simon Collins, Sanctuary Manager at the Rotokare Scenic Reserve Trust (RSRT) in the country’s Taranaki region.

“You catch them, you put bands on their legs, you possibly take a blood sample, and then they get stuck in a box. They might travel by boat or plane or whatever, and then the box opens and they just go berserk,” he describes.

The diminutive, nectar-eating hihi (Notiomystis cincta)was wiped out from New Zealand’s mainland more than 130 years ago when European colonizers destroyed vast swathes of their habitat and introduced predators like cats, rats, stoats and possums. The species survived only on one small offshore island, Te Hauturu-o-Toi.

Thanks to the efforts of conservationists, hihi have been successfully reintroduced to six highly protected reserves, and the population has grown to several thousand. Successful translocation will be crucial to growing the species’ numbers to more tenable levels, says Collins.It’s a conservation tool that’s become increasingly successful in recent decades, and New Zealand is a world leader in the craft.

But it’s always a challenge, says Oliver Metcalf of the Zoological Society of London (ZSL) and Imperial College London (ICL). One of the biggest hurdles is monitoring after release, he explains, “which is really important in understanding how and why these projects succeed, so it can be repeated going forward.” 

HOW’S THE NEIGHBORHOOD?

Until now, most monitoring has relied on attaching radio transmitters to animals, as well as physically monitoring them. But the transmitters can be “quite intrusive for birds,” says Mhairi McCready of ZSL and the Hihi Conservation Trust. What’s more, both methods are time- and resource-expensive, and they risk changing the birds’ behavior. “So they may behave differently because they’ve got an antenna on them, or because they know we are watching them, and then what we’re recording may be slightly skewed,” she says.

That’s why, in a study just published in Methods in Ecology and Evolution, scientists at ZSL and ICL, alongside conservationists from the RSRT, pioneered a new method of monitoring hihi reintroductions. They used remote acoustic recording devices to ‘eavesdrop’ on 40 young hihi that were reintroduced to the Rotokare Sanctuary in 2017.

“It’s a bird-friendly way of monitoring,” says McCready. “You put your recorder out, and you leave it there, and you just listen in to what they’re doing naturally.”

The devices recorded ‘happy’ hihi calls, which sound “like two marbles clanging together,” describes Metcalf. The researchers were able to observe how the calls changed from a random distribution to a settled home range, suggesting that they had established their own territories and that the reintroduction was successful.

“At the start of the study, the birds didn’t really have any preferred areas as they explored Rotokare,” says Metcalf, “but after a month it was far more likely to record hihi calls at recorders close to water, showing that they had started to settle onto territories close to water.”

“We also found that they traveled around, and you can see those patterns following the fruiting and the flowering of the different plants as well,” adds Collins. 

SAVING SWEAT

The new method is tipped to save researchers and volunteers serious time and energy in their monitoring efforts. Most remnants of forest in New Zealand are on rugged bits of land “that were too steep to be something else,” says Collins. “So existing monitoring [without using the remote devices] is, you go out with a pair of binoculars, a notebook and your lunch, and you’re all day going up and down hills.” Given that organizations like the RSRT are “quite reliant on volunteer time,” it can be difficult to engage enough people to do the work required, he says.

The new tech doesn’t do away with human effort entirely. Metcalf said he still had to carry recording units up and down steep hillsides and through dense forest in order to place them in a grid pattern across the terrain, “but a lot less often than if I was trying to directly count the birds myself.”

Processing the data was another challenge, he says. “We had so much that it was impossible to listen through it all. So we used a machine learning technique to recognize the hihi’s call, and then a statistical technique – dynamic occupancy modeling – to associate the areas we were most often recording hihi calls from and the habitat types of the areas the birds preferred.”

PRECIOUS PROTECTION

Teaching the machines to recognize the hihi’s call was challenging, particularly given the complexity of the New Zealand forest’s soundscape, said Metcalf. But having succeeded, the team is now quite confident that this technique would work with a broad range of species that call regularly. The technique could also be used for general monitoring, “to see what species are present in a place,” says McCready. 

From here, it’s likely that the scientists will be able to start using the data to locate trends and ultimately predict populations’ movements and dynamics over time, “so we can start to do the relocation much more precisely and cost-effectively,” says Collins.

For the hihi, these efficiencies may make the difference for their survival. “There are still only seven hihi populations on the planet,” says Collins. They’re also an age-old indicator of forest health in indigenous Māori culture, he adds. “So it’s a really precious thing that we’re endeavouring to protect.”