Monitoring bats in Whirinaki

By Neil Robert Hutton – Community Ranger, Department of Conservation

Christmas was fast approaching and most people were anticipating Santa’s reindeer; however, in the Whirinaki Te Pua-a-Tane Conservation Park, Department of Conservation rangers were on the lookout for another sort of flying mammal.

A short-tailed bat being released in the Whirinaki Te Pua-a-Tāne Conservation Park.

As night fell, I lay on the forest floor, vaguely aware of giant beech trees towering overhead like pillars in a dimly lit gothic cathedral.  Kākā screeched in the canopy above and kiwi calls rang out from the gloom as my colleague, DOC biodiversity ranger Sarah Wills, quietly and carefully raised her black netting into position.  Around me, volunteers and other rangers spread out around the net like some sort of conservation special forces team: faces focused and ready for action. Nothing happened.  A few minutes more and still nothing.  Then, suddenly, there was a light movement in the netting and Sarah said “BAT!”  The still forest floor erupted in activity as the team sprang into action; we had just caught our first short-tailed bat.

The bat catching team getting ready to go out for the night.

Every December for the last five years, Sarah and her team have spent two weeks monitoring the short-tailed bat population in the Whirinaki Forest.  Short-tailed bats are one of the only native terrestrial mammals of New Zealand Aotearoa.  Once wide-spread, short-tailed bats are now reduced to a population of around 50,000 scattered in 13 principal locations. The Whirinaki Te Pua-a-Tane Conservation Park near Murupara is one of these last strongholds.  The Whirinaki remains a sanctuary due to its prodigious supply of massive trees which the bats need for their roosts.  Sarah’s monitoring programme was established to identify these ‘roost’ trees to better manage DOC’s predator control measure in the park.  Since 2013, 17 major roosts have been located including one with an estimated 5,000 female bats, which is affectionately dubbed ‘Swiss cheese’ because of the many entrance holes.

Bat roosts are not particularly easy to find in a 70,000-hectare forest.  I tagged along on the most recent monitoring trip to learn about the process – all the way from bat in the net to bat in the roost.  In preparation for the monitoring trip, Sarah placed over 20 listening devices in key locations around the forest.  The devices are sound activated and only react to the two frequencies produced by our native bats.  Based on the results from the listening devices, Sarah knows what areas have the most bat activity and therefore what are the best places to catch bats.

The team setting up the mist net. The net is almost invisible in the image because it is so fine.

With the information gathered from the listening devices, Sarah plans out where she will set her nets.  Known as ‘mist’ nets, these nets are specially designed to be invisible to both birds and bats.  The net itself is 12 metres long and amazingly delicate; it is strung horizontally across an opening in the forest.  As the bats fly through the clearing they hit the net and fall into baggy pockets that hang loosely along the net.  As I help set up the net, Sarah explains that netting bats can only occur when there is little to no wind because if the net moves, the bats’ echo-location will pick it up.

Net in place, the team prepare for the bats.  Each team member has a responsibility.  Some are ready to carefully extract the caught bats from the net, others clutch soft cotton bags used to safely hold the captured bats.  Still more team members wind bird squeakers over and over to produce attractive little squeaks and others mind the ends of the net in case it needs to be lowered to extract a bat.  The net is raised, and everyone sits silently anticipating the first catch.

Hayden and Sarah work to untangle a short-tailed bat from the net.

Huge beech trees tower overhead in the dusky forest.

The goal each night is to catch four to five female bats. Catching the bats in the net is one thing, but getting them out is another.  None too happy about being caught, the little bats have ferocious tempers and will bite anything they can get their razor teeth into.  Focus, patience, and a tolerance to the occasional nip are all prerequisites for the team members detangling bats.  Once freed the bats are stashed in cotton sacks for safekeeping until the catching phase is done.

How does the team go from having netted females to knowing where those females are roosting?  Once Sarah has enough females, she places radio transmitters on them.  The tiny transmitters will let Sarah track the bats and locate where they are hiding out in the daytime.  The bats must weigh at least 13 grams to be eligible for a transmitter.  The transmitters are delicately glued onto the back of the bats to avoid hampering their flight.  I’m amazed at the dexterity required as Sarah deftly prepares the bats for their transmitter.  The team all sit quietly by as if watching a great performance – silently following Sarah’s every move.

Dave removes a bat from the net as Hayden watches the net in case another bat flies into it while Dave is working.

Sarah collecting a bat listening device

Once the transmitters are secure, the bats are free to go.  It is now well past 1am and the team can begin the long tramp back down to the campsite.  As we make our way slowly through the night-time forest, millions of stars flicker overhead.  The next morning the team sets out with radio aerials to try and find the signals of the bats we put transmitters on the night before.  The aerials can pick up a signal from a few kilometres away but finding the source of the signal is more of an art than a science.  Triangulating the signal to find the source (and hopefully a roost) sounds fancy but really it is just a whole bunch of walking around in circles in the bush!

Several hours and quite a few kilometres later, the signal is starting to get louder.  The roost must be close!  beep… Beep… BEEP… surely it must be around here somewhere?!  Then we smell it.  I’m not quite sure how to describe the smell of thousands of bats all huddled together in a hollow tree slowly filling it with guano but it was certainly a sharp smell.  Quietly, so as not to disturb the resting bats inside, we examined the tree for entrances.  A great big hole halfway up the trunk with guano spilling out the bottom looked to be the main access point into the roost.

The beech tree, home to the bats.

We set up an infrared camera on the roost exit to record roughly how many bats were leaving the roost at night.  This will help give Sarah and her team an idea of how many bats might be in this new roost.  Sadly, the camera footage often also captures things like stoats going into the roost to predate bats. But with another roost found and an extensive predator management programme, hopefully short-tailed bats will still be flying the Whirinaki skies next Christmas.

This bat was fitted with a transmitter. The team later followed it’s signal to a new roost which the project hadn’t discovered yet.