The Great Barrier Island Bittern Hunt - the shy giant of the wetlands maintains a low profile on GBI

by Amelia Geary

Environmental News Issue 29 Summer 2012

When talking to people about ‘bitterning’, usually the first question people ask is: what’s a bittern? Bitterns aren’t as conspicuous as the tui or as loud as the kaka on Great Barrier and only a few people have ever seen one. They are a member of the heron family and live in tall, dense beds of raupö and reeds in freshwater wetlands.

Bitterns are large, standing over half a metre high, and males can weigh 1.4kg. Given their size, it seems strange that so few people have laid eyes on one but that’s because they are incredibly cryptic. Their feathers are different shades of brown which appear almost striped. This is important for camouflage because a defence (and hunting) mechanism they employ is to stand tall, with their beak facing the sky, swaying in the breeze like a piece of raupö. When they do this they are almost invisible, to predators and prey!

Bitterns nest in dense stands of raupö and reeds surrounding lakes and in swamps. Calling from within the reeds, males advertise their territories by giving a deep resonant booming call – a noise that sounds uncannily like someone blowing repeatedly across a glass bottle. Booming occurs all year round but peaks during the breeding season, from September to November.

Bitterns are nationally endangered; their threat status is only one category below ‘nationally critical’ which is reserved for species such as takahë and kakapö. Their populations have declined in the face of drainage and ‘reclamation’ of wetlands. Introduced predators are almost certainly having an impact on numbers as well. Bitterns are an Australasian species and breed in Australia and New Caledonia too but New Zealand has the best population with a national estimate of fewer than 1000 birds. Unfortunately this is little more than an educated guess, their cryptic nature making it extremely hard to monitor these solitary wetland birds.

No one has ever done a thorough survey of the Barrier bittern population so we don’t know how many we have! However, using the information we do have, it seems reasonable that we could be home to a significant number of these secretive locals. Great Barrier with its lack of ferrets and stoats and large raupö wetlands could potentially be a national stronghold for the species. In order to find out, DOC with the assistance of the Great Barrier Island Trust and enthusiastic members of the community has started to listen out for them, attempting to develop an accurate population estimate.

In order to get an estimate of numbers, people sit at predetermined locations half an hour before sunset to listen for booming males over three consecutive calm nights. This year a total of 18 listening posts are located at Okiwi Station, Kaitoke Swamp and Policeman’s Swamp as well as a site at Medlands and at the bottom of Blind Bay Road. The first round of listening occurred in September with a highlight being the sighting of a bittern feeding at Medlands causeway (see photo), two nights in a row!

So far, most of the booming has been recorded roughly half an hour after sunset. The best night of listening was September 19 when six different bitterns were recorded. Interestingly, Policeman’s swamp – a hotspot last year – was quiet in September, but it’s hoped that after multiple nights over multiple months, we’ll have a good population estimate by Christmas!