The sound begins with a sharp inhale—the kind you take before you’re about to sneeze. Second comes a bassy wooom, akin to blowing across the top of a glass bottle.

The soundmaker is close but invisible, concealed among thick yellowing raupō. The sound repeats three times, then silence.

Department of Conservation (DOC) ranger Allanah Purdie checks the time on her phone and notes it down: 7.34pm. Number of booms: three. She’s in a deck chair, rugged up in beanie and puffer jacket, on the edge of the Halswell River in Christchurch. It’s crisp but clear, a pink spring sunset blooming in the west.

Purdie’s here for the boomer—an Australasian bittern or matuku. A master of camouflage, his plumage of streaky brown and cream blends into the reeds, especially when he stands stock-still, neck elongated. But in the evenings of spring and early summer, his boom reverberates through the wetland. It’s his way of saying “I’m here” to both prospective mates and rival males. A foghorn in the impenetrable scrub.

It’s a signal Purdie and other conservationists are tuning in to in an effort to understand this notoriously elusive wetland bird. As the breeding season—the booming season—ramps up, clipboard-wielding volunteers are listening in wetlands across the greater Christchurch area. Audio recording devices are deployed. Booms are triangulated, pinpointing males on a map. Over the years, a picture emerges: a handful along the Waimakariri, a few at Hart’s Creek on the edge of Te Waihora, and always, a lone boomer at the Halswell River.

“I would love to know if it’s the same individual,” says Purdie. She wonders, too, whether his booms have ever landed him a mate. At the end of each season, his calls eventually give way to the wind in the reeds.

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Traces of bitterns in te ao Māori are today as elusive as the bird. But glimpses from a time when matuku were abundant—in 19th century texts, waiata or traditional histories—show them woven into the spiritual and mythical world.

Iwi from the North Island’s west coast tell the story of the great navigator Turi, who brought matuku here on the waka Aotea. Turi’s people settled on rich, fertile land in Pātea and used the bird as an alarm system. Left on guard in an empty pā, it would call at the first sign of strangers. The booming siren alerted Turi and his people to danger, and also tricked outsiders into thinking the pā was occupied.

As matuku spread across Aotearoa, their calls took on different meanings. To some, “kau kau” heard in flight could portend floods. “Hū hū hū” resounding across the marshes might be the bittern calling on its ancestors, or the anguished moans of atua trapped on Earth. In some old stories, a murderous ogre called Mātuku turned himself into a bittern and hid in the swamp to elude a foe.

The matuku’s otherworldly booms spoke of the spirit world to many. “Our people were a little bit frightened of them,” says Colin French of Te Uri o Hau, a hapū from the northern Kaipara region. “They were eaten, but they weren’t a common food. Resources had to be really, really squeezed before they started eating those.”

Most often, the bittern’s boom is linked with grief. In the waiata ‘Whakarongo e te rau’, attributed to Timotu from Ngāti Ruanui, a lamenting singer compares themselves to the bittern:

E maero au nei,

E kaahu e keo ana i te waru.

Kei te matuku e hu ana i te repo “hu hu”

I am an angry spirit,

A hawk screaming in winter

A bittern hooting in the swamp.

Now more than ever, the bittern’s story is one of sorrow. This wetland bird that once flocked in groups 100 strong is nearing extinction. The decline began in the late 19th century. As settlers drained wetlands of water, they emptied out the bitterns too. At Lake Rotorua, in 1937, a resident reported flushing 44 bitterns from a single patch of raupō. Fast forward to 1980: a lone bird is recorded booming there. None have been seen since.

The decline has accelerated in the last 40-odd years. Today there are fewer than 1000 bitterns in New Zealand, making them ‘nationally critical’—the same threat category as kākāpō. But unlike kākāpō—where every individual is meticulously tracked and managed—the bittern has proven slippery. Unravelling the bittern’s story—its way of life and precipitous decline—has meant finding new ways to listen, to see.

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Bitterns are shapeshifters. They can hunch up in a kiwi-like ball, or elongate, “like a proper heron”, says Emma Williams, a science advisor at DOC. When they’re recoiling to strike at prey, they adopt what she calls a “dinosaur shape”. “They puff up—their beaks are open and they’ve got mean eyes. Their neck goes from really thin and slender to like a frilled lizard.”

Williams has a PhD in how to find and follow cryptic (hard-to-detect) species. Her thesis includes a section titled: ‘Developing a monitoring method for a species that looks and behaves like a plant’. For several years, Williams lived in the wetlands near Waipukurau in central Hawke’s Bay with her dog Kimi, who she trained to sniff out bitterns. On still evenings, she could drift silently in her kayak past the raupō beds, so close to bitterns she could feel the intense rush of air from their booms.

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Over five years, Williams caught 10 male bitterns on the main lake in the wetlands, affixing radio transmitters between the birds’ wings like backpacks. She named the bitterns after old-school crooners: Bing Crosby, Barry White, Frank Sinatra. Using an antenna and playing hot-and-cold in the field, she tracked the boomers. Their routines and way of life came into focus: leave the lake in autumn. Relocate to neighbouring farms for winter. Return to the booming territories, on the lake, in spring.

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The bittern’s big surprise came in the summer of 2015 and 2016. Williams spent that season flitting between Canterbury, the Bay of Plenty, and the Waikato, catching bitterns. This time, she had satellite tags instead of radio transmitters. Rather than relying on a handheld antenna with limited range, the bitterns’ location pinged off satellites once a week.

Williams was downloading the satellite pings for a bird tagged at Te Waihora/Lake Ellesmere, in Canterbury, when she noticed something odd. “I was like, ‘Oh! It’s communicated via a satellite that’s over Blenheim. That’s bizarre.’” She double-checked her data. Then she checked the Waikato birds “and they weren’t where they were supposed to be either”. Like Williams, the bitterns had been on the move. “It identified this beautiful network of wetlands.”

The data crystallised a startling realisation: bitterns could have been double-counted. And you couldn’t just focus conservation efforts on one wetland and expect a boom in boomers. “Their bittern is also your bittern, and your bittern, and your bittern,” Williams says. “And if all of you aren’t protecting them, then we’re in trouble.”

The bittern’s nomadic tendencies are also a sign they’re in bad shape. Williams speculates the need to roam comes down to two factors: food and females. “Perhaps, they’re exploring wider to find where the females are,” she says. “Or they’re competing for food and getting pushed out, exhausting food sources altogether. Or they literally can’t access food at the site due to sedimentation and are moving somewhere else to find it.” Since bitterns hunt by sight, they need clear water to snatch eels and frogs from freshwater shallows. The water level has to be just right, too: between 15 and 25 centimetres is ideal for finding a feed.

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In early September, a farmer found a starving bittern on a roadside near Te Anau. When the bird arrived at Dunedin Wildlife Hospital, he weighed just 600 grams. A healthy male bittern should be at least 1.4 kilograms. He was “very weak and emaciated”, says veterinarian Lisa Argilla.

Argilla and her team kept the bittern warm in an intensive care unit, and hydrated him with intravenous fluids. “Then it was a slow process of introducing food,” Argilla says. If a severely malnourished person—or bittern—is fed too much too quickly, they can die within a few days.

By mid-September, the bird was much stronger. Argilla and her team moved him to an enclosure with native vegetation for cover. Hospital is a stressful place for any bird, but especially so for a secretive bittern. He’d only be disturbed twice a day, for weigh-ins and a calorie-rich fish smoothie. He’d puff himself up to appear bigger and lash out with his sharp beak—necessitating safety goggles for anyone getting close. But bittern beaks aren’t the only hazard: “The smoothie made him poop really gross. It’s stinky, almost diarrhoea-like poo,” says Argilla.

Although the bird seemed to be on the mend, the threat of organ failure lingered. In late September, the bittern developed a swollen foot from gout, a sign his kidneys weren’t working. Two weeks after he was admitted, the bittern succumbed to kidney failure. Death is something you deal with often as a wildlife vet. But it’s still “an extremely sad outcome”, Argilla says. “Nothing our team could do would have saved this beautiful bird.”

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Starving bitterns don’t always make it to hospital. Williams and the DOC team have autopsied dead bitterns and found depleted bone-marrow fats—a sign the birds are digesting themselves to stay alive. To figure out just how many are going hungry, and whether this is a new problem for bitterns, PhD candidate Vanessa Kennard is deciphering messages inscribed in feathers.

At a lab on the University of Canterbury campus, Kennard brings out a Ziploc bag of speckled plumes, some downy, others sleek. If you lift a feather to the light and angle it just right, distinct bars are visible, like rings in a tree. Each bar represents about a day of growth, making each feather a window into the daily life of a bittern over one to two months—the time it takes a feather to fully form.

Kennard has amassed her collection of feathers from 84 bitterns—both historic museum specimens and recently deceased birds, covering a timespan from 1886 to 2023. Today’s bitterns have haphazard growth bars compared to their ancestors, her analysis has found.

The birds are struggling to find kai, and their hunger is etched in their feathers.

The feathers have more secrets to divulge. Kennard takes tiny snippets to analyse their chemical signatures. Bay of Plenty bitterns have carbon signatures indicating they move around a lot. Nitrogen signatures tell a more complex story: they have shifted substantially over the decades in bitterns from Canterbury and Bay of Plenty—regions with depleted wetlands.

This change in nitrogen could be caused by three things, Kennard explains: what the bittern is eating, where it lives, and whether it is consuming itself to stay alive. She’s planning new experiments to tease out what’s happening, but suspects a combo may be in play—especially with nitrogen running off farms laden with cow poo and fertiliser, flooding our freshwater and fuelling algal blooms that alter food webs.

“The bittern’s issues are the big issues of New Zealand,” says Williams. “They’re our water quality, the connectivity of our rivers.”

Ninety per cent of New Zealand’s original wetlands have been cleared. Suitable inland sites, in particular, have disappeared. “We’ve lost a lot of habitat—we know that—but the wetlands we’ve got left are still under threat.”

Bitterns are the canary in the swamp, and they’re telling us that our freshwater is in a dire state. Tēnā kei te repo, he matuku noho puku. In the marsh the bittern sits in silence.

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The bird was standing motionless on the road in Motukarara when Peter Langlands rounded the corner in his car. There was no time to swerve, but he remembers a fleeting thought: Is that a pūkeko? Upon impact, Langlands realised the bird he had just run over was, in fact, a bittern—a bird he had become intimately acquainted with over recent years. He slowed and stopped, trudging up the roadside to retrieve its body. “I was absolutely in shock,” he says. The bittern was a juvenile, close to fledging. “It was just like a rag doll. It was skin and bones. Totally emaciated. It was surreal.”

Langlands saw his first bittern as a teenager on a birdwatching trip to the Canterbury high country in the mid-80s. A flurry of feathers had rocketed out of the rushes at Lake Alexandrina, “like a jack-in-the-box”, Langlands remembers. He became particularly enamoured with the species while working as a fly-fishing guide. “The bittern is a metaphor for the wilderness of wetlands,” he says, labelling its eerie boom “the call of the wild”.

In 2006, he wrote an article for Trout Fisher magazine on bitterns, and the iconic matuku trout lure that was once crafted from bittern feathers. (Historically, Māori fishers, too, used the feathers in lures.) The yarn caught the eye of Colin O’Donnell, a DOC scientist, and Langlands sidestepped into bittern conservation during the fishing off-season, helping to compile a national database of bittern sightings—and deaths.

Most bitterns in the database die from one of two things. “It’s an arms race between collisions and starvation,” Williams explains (noting that other causes of death could be significant, but we may not be findings these birds’ remains).

By destroying the bittern’s homes, we’ve set them on a collision course with cars, trains, powerlines and buildings. In October, two bitterns were killed metres apart on State Highway 12 near Dargaville—Te Uri o Hau’s whenua. “That was a sad thing,” French says. “The buggers had been out there eating lizards which had been out on the road, warming up.”

French spent 10 years working as a roading contractor. “Just about every single drain in the Kaipara, I see them. I’ve got my bittern eyes on—I can pick them up from anywhere. I can see them blink. They stand still and they stretch their neck up and try and look like a stick and I can see the buggers blink.”

He warns drivers that matuku won’t flee an approaching vehicle. “You’re actually going to have to physically hop out of the car and push them off the road.”

And increasingly, bitterns are gravitating to roads. According to Langlands: “They’re spending a lot of time feeding on roadside margins in ditches and drains. Because there’s so little wetland habitat left.”

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The drone lifts off before first light and hums 60 metres above the scrub, proceeding on its programmed zig-zag flight above the Waihī Estuary. Caley and drone pilot Hamish Kendal huddle around a monitor, watching intently as a black-and-white bird’s eye view unfurls across the screen. The drone is equipped with a thermal camera; warm bodies appear as glowing white pinpricks.

While male bitterns have given up some of their secrets—thanks to their booms making them easier to locate and catch—females are more elusive. “We have no idea about female numbers,” says Caley. He suspects there is an overabundance of males. To illuminate the secret life of female bitterns, Caley has turned to tech.

Sometimes the white dots are unwelcome creatures: a huge Norway rat scurrying through the wet tussock, a family of five ferrets. But most are birds. White dots clustered together are usually pūkeko, pheasants or waterfowl—a bittern will be alone. When a single white dot is spotted, Kendal switches the image to full colour, lowers the drone and zooms in to locate a brown, bird-shaped splodge. But more often than not, the splodge turns out to be a harrier.

At 7.40am, they spot the first bittern, slinking along the edge of a pool, its splayed toes gripping the grass.

It slides into the shallows and stands stock-still, neck outstretched, intently scrutinising something in the water. Fish flash at its feet. The drone beeps—low battery.

Before the drone, field contractors spent hours and hours in waders and kayaks, wandering through the bog looking for females alighting in the raupō, fingers crossed they’d stumble upon a nest. The odds were not in their favour. Even with the drone, Caley and Kendal have found just three nests over two summers, including one at Waihī Estuary last summer. Caley waded through the wetland to install a remote camera so they could closely watch mum, her three eggs and one chick.

“We had somebody check footage every hour,” says Caley. “They’d check on the nest and share photos in a WhatsApp group. We were really excited.” A week in, the eggs had hatched: four hungry bittern chicks huddled on the nest. Then “we got this image: a harrier swooping down”. It snatched one of the chicks in its talons. “We progressively saw each chick taken by a harrier. It was devastating.”

This same scene played out at all three nests. “Harriers are becoming a real problem,” says Caley. The native raptors have flourished in the disturbed environments humans have created. To save the bittern, we may need to cull the kāhu—at least in wetlands with vulnerable nests.

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Bittern conservation is tricky, Williams says, because we need to start managing their dwindling population at the same time as we’re learning the most basic things about them—like, how many bitterns are left? What’s their natural lifespan? And what’s the most important threat to tackle first? “If we wait for that information, it’s too late. We have to start with something and be learning as we go,” she says.

But the challenge ahead is immense. Aside from predator control and getting weeds like willows out of wetlands, there’s a bigger battle: between water management for the agricultural sector, and water quality for wetlands. “Bitterns are basically in the trenches,” says Langlands. “They’re in this zone where we’ve got big landscape changes happening, big changes in water quality.”

Caley sees a middle-ground solution. Instead of manmade ditches and drains that funnel water away as fast as possible, he envisages a slower way to manage water for both people and wildlife: wetland corridors connecting coastal and inland sites, ki uta ki tai, from the mountains to the sea.

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There’s one place in New Zealand where the bittern are booming—or at least, doing okay. “I’ve heard the most booming on the West Coast of the South Island,” says Caley. “In one sound file, we captured 64 bittern sequences in 15 minutes.”

The birds on one patch of that coast, Karamea, are lovingly chronicled on Facebook by an amateur photographer who prefers to remain anonymous. In winter especially, she has noticed the birds frequent a particular paddock of humps and hollows with drains full of eels. Mostly, the birds hide—or try to, since their brown-and-white camouflage doesn’t serve so well against dairy-farm green. One time, she observed a kerfuffle over kai: “I saw these two bittern, fluffed up, and one had an eel and one didn’t. And they seemed to be chasing each other around for it,” she says.

Over the years, different bitterns have come to feed in this paddock. There was Limpy, who had a limp, but who was sadly hit by a car a couple of years ago. Quinny, who hung out by a bridge, and Patch, with lots of cream colouring “like patchwork”. The latest two are Smoothy and Bumpy, named for their contrasting appearances: “One is so beautifully smooth with lovely feathers, and the other one is quite shabby-looking.”

Across Aotearoa, bird-inclined laypeople are discovering bitterns in their backyard. “There’s a lot of stuff the community is doing which is really fundamental for this species,” says Williams. “We can’t do this alone.” From Northland to Otago, citizen scientists are documenting booms for a project dubbed “OK Boomer”. At Matuku Link in Auckland, volunteers are restoring bittern habitat by ripping out willows, trapping predators, and growing and planting natives suited for a swamp. Langlands is leading the creation of a Bittern Conservation Trust, bringing together all those “smitten by bittern” to advocate for the species.

“We’ve pushed the species right to the threshold. Death by a thousand cuts,” he says.

Earlier this year, two emaciated bitterns were rescued from South Auckland in the space of a week. They were found close to two maunga jointly known as Matuku-rua. The twin maunga reflect two moods Māori associate with the birds: Matuku-tūreia meaning ‘standing and watchful’ and Matuku-tūruru, ‘restful’.

After weeks of care at Auckland Zoo, one of the bitterns died. The other, a female, stacked on the grams to reach a healthy weight. She was gifted the name Te Awanui by local iwi Te Ākitai Waiohua, equipped with a tracker backpack, and farewelled with a karakia, before being released into a healthy wetland at Lake Rotokawau, on the Poutō Peninsula of the Kaipara Harbour.

French says this part of his hapū’s rohe is a “bittern stronghold”. Here, Te Awanui seems to have lost some of her innate wariness. Every now and then she’ll wander up a drain to his cousin’s place and look in the window. Then she’ll disappear again—but not completely out of sight. Dots on a tracking map show the bird exploring her new home, providing a rare peek into the life of a female bittern.

French is confident Te Awanui will find a mate on Poutō Peninsula. He likes to think of her, on the lake, feasting on frogs.