Australasian Bitterns in Serious Trouble

by Emma Williams


Wetlands in New Zealand are under-represented and sadly under-appreciated by the vast majority of New Zealanders. Yet, most duck hunters and farmers will know these secret, fertile refuges are important habitats for many species and are packed full of life. Indeed, only those who have made the effort to access and spend time in such challenging wetland environments will know some wetlands are home to a special secretive and cryptic bird.


Water skills required: Large cage traps for bittern and other equipment are all transported around Lake Whatuma near Waipukurau, by kayak.
Photo by John Cheyne


A bird so evasive and shy that little is understood about them, and what we do know is based on a few opportunistic observations, or the studies of closely related species overseas.

In spring the males seduce their females with deep-booming calls that can be heard up to 4kms away. Yet, often these calls are mistaken for the distant bellow of a cow and even those lucky enough to live on the fringes of wetlands do not know that this species exists.

Have you heard this bellowing call? Often likened to blowing air across the top of an empty bottle, or a low Whoomp noise. If so, did you know that what you have on your doorstep is the rarest bittern species in the world? The Australasian bittern (Botaurus poiciloptilus).


The Australasian bittern, matuku herepo (botaurus poiciluptilus) hanging on in the wetlands of GBI. Photo:

If you are one of the few people who regularly sights a bittern, or has been serenaded to sleep by the deep-baritone booming call, then you are fortunate. For who knows how much longer these crooners will persist. Australasian bitterns are in serious trouble. There is no doubt about it. Ranked as Endangered by the IUCN (International Union for Conservation of Nature), Australasian bitterns are only found in New Zealand, New Caledonia and Australia.

These bitterns are so rare they have a higher threat ranking than the blue duck (Whio, nationally vulnerable), kokako (at risk) or the North island brown kiwi (nationally vulnerable). Yet the bitterns’ wetland habitat does not receive anything near the protection of these three species.

A decade ago there were thought to be less than 1000 adult bitterns each in New Zealand and Australia, and fewer than 50 in New Caledonia. However, recently Australia has lost large numbers of its population, and the New Caledonian population is thought to have vanished. New Zealand is therefore an important place for bitterns and the survival of the species will depend upon how we manage our wetlands.

There are several factors limiting bittern populations in New Zealand. For starters, bitterns rely upon freshwater wetlands to breed and feed and as we know, 90 percent of these habitats are gone. Worryingly the wetlands that remain are still under threat. Additionally bitterns are ground nesters, making them highly susceptible to predation by introduced mammals. Female bitterns are smart and sneaky. In order to give their chicks the best chance of survival they build their nests on floating platforms that are well hidden in the thick vegetation. Unfortunately, chicks are less savvy and have a tendency to be noisy and smelly. Once detected, these Kentucky-Fried-Chicken-sized morsels will be easy prey for any resident cat, ferret, stoat, weasel, or rat. Even adult bitterns are not safe, as these can be on the menu for cats as well.

So what about their food? Surely with so few bitterns around these days there will be plenty of tucker to share? But alas, no! Bitterns feed predominately on eels which are also considered to be under threat. Luckily as opportunistic feeders, bitterns can take a range of alternative prey items including small fish, insects, and frogs. However they need to see through the water to be able to spot their prey – something that becomes difficult in degraded wetlands where water clarity is low.

Water depth is also a problem for bitterns, mostly because humans have a tendency to want to control and manipulate wetlands to suit their own purposes. For example, wetlands are often used as holding tanks to keep flood-water off farmland and other nearby property. This causes sudden fluctuations in water depth that can wash away bittern nests or completely dry out areas so that predators have easier access. Landowners often dig deep drains so they can conveniently channel the water away from their land. Sadly in these deep channels, bitterns have the same chance of catching their prey that a drunk man has of catching a teddy bear in the pub with a crane claw machine.

In particular, messing with water levels can affect the survival of young bitterns that are inexperienced and rely upon having access to easy prey. For example, naturally, most wetlands would dry out over summer, often creating small, shallow pools where fish become trapped. The timing of this coincides with when young bitterns have left the nest and are starting to learn to find food themselves. And what could be better for these gangly, clumsy feeders that an easy to access all-you-can-eat-watery-buffet? When we artificially flood wetlands or channel water into drains we interfere with these natural processes, affecting the survival of bitterns and their prey species.

For us to be able to save the bittern in New Zealand, we need to know which wetlands are important to them, what their greatest threats are and how different wetland management methods affect the survival of bitterns. Unfortunately, as bitterns are so cryptic, we first need to be able to find them.

This year we are working on this problem by testing several bittern monitoring methods on Lake Whatumâ, near Waipukurau, Hawke’s Bay. We intend to test different methods that have already been used successfully on bitterns and/or other cryptic species internationally. What we need to know is how well these methods detect our bitterns and whether the number of birds detected by each method actually relates to how many bitterns are on the lake. To know this we need to catch as many bitterns as possible and mark them in a way that will tell us if each individual bird was present or not during monitoring and then if it was present, was it detected?

Emma setting two bittern cage traps which incorporate a treadle/door release and mirror to attract male bittern inside the trap. A recorder playing the booming call of the male is placed on top of the traps to initially attract the male to the site. Photo: John Cheyne

To mark birds we attach a radio transmitter to each captured bittern using a harness that looks like a backpack. Each transmitter emits a radio signal on a unique channel that can be heard as a beeping noise on aTR-4 receiver. Harnesses are designed to have a special ‘weak-link’, which allows them to break and fall off should the bird become entangled in a life threatening situation. Putting radio transmitters on bitterns is the only way we can locate and identify individuals when they are hidden in thick Raupô. This means when we run each monitoring method we can check where our marked bitterns are and see if they have been detected.

The batteries on our transmitters will last for about a year and a half. In that time we are also hoping to learn more about bittern behaviour and answer questions like: Where do bitterns go after the breeding season? What habitat types are important to bitterns for feeding and breeding? How big are home range sizes? And do bitterns come back to the same sites every year to breed? These are basic but important questions that at the moment nobody has answers for…


Australasian Bittern facts:

Breeding season = Aug – May
Number broods per season = 1
Mean clutch size = 4
Egg colour = Olive-brown/
Light green-blue
Egg laying = Aug – Dec
Incubation length ˜ 23 days
Age at fledging (mean) ˜ 49 days
Adult weight (males) = 1400 g
Adult weight (females) = 900 g


Emma’s Profile
Emma Williams is a PhD student at Massey University where her work specialises in developing monitoring methods for cryptic species (species that are rare and difficult to detect). Originally from Stoke-on-Trent in England, Emma came to New Zealand in 2007 and started work for the Department of Conservation as a research assistant in Fiordland. Now a New Zealand resident, Emma began her bittern work in 2009 as part of a Department of Conservation contract, and very quickly became smitten with the species and the challenges experienced while working with them. In 2010 she took the bittern project on as a Masters of Science which quickly ballooned into what is now her PhD. She currently lives in a camper van at her study site with her field assistant, a black Labrador called Kimi.

Ducks Unlimited’s contribution…
This year, Ducks Unlimited supported Emma’s research with a substantial grant of $3200 which allowed her to purchase 10 transmitters. These are to be attached to individual bitterns at one of her study sites (Lake Whatumâ), allowing her to identify and re-find these birds even when they are hidden in the thick Raupô.
In order to attach these transmitters, Emma must first find out how to catch these birds. To do this she has been working closely with DU president John Cheyne, to develop a successful capture method. This is no mean feat as the only method that’s been successful in the past involved a helicopter, a resource that few students have access to, and is obviously somewhat stressful to the birds. Emma and John have been working closely with bittern researchers overseas to develop alternative capture methods that are successful, cheap, easy to use and most importantly less terrifying for the birds.

If the project goes ahead as planned it will be the first time such a large number of Australasian bitterns have been captured and studied intensively. It’s hoped results will be useful for wetland and bittern conservation nationally. Emma is very grateful for the support provided by Ducks Unlimited and looks forward to presenting DU with some results early next year.

Bitterns have been discussed in The Environmental News on previous occasions (2006, Issue 8; 2010, Issue 23; 2013 Issue 30), and members of the GBI Environmental Trust participated in surveys of Bittern numbers by the Department of Conservation in September and November 2012. The results were published by DOC (Geary, A., Corin, S. & Ogden, J. (2012) Australasian Bittern Survey. Monitoring Report . GRBAO-22380-2012. Department of Conservation, Great Barrier Island). The tentative conclusion in that report is that Great Barrier has a small breeding population of bitterns – possibly three pairs – in the Kaitoke (swamp) catchment (including the swamp in front of the police station – subsequently burned in January 2013) . Although there were sightings in the Okiwi area, there was no indication of breeding elsewhere. If you have any sightings of bittern on GBI please forward them (with location and date as close as possible) to, or phone (09) 4290980.


Environmental News Issue 34 2015