Date: 22 April 2010
Department of Conservation (DOC) staff from Ohakune have recently been confronted by the damage feral cats can do when left to target our native wildlife. A survey of the resident bat populations on Southern Ruapehu gave staff a vivid reminder of this whilst out on their nocturnal travels.
For them the forest can be an eerie place to be at night. Silent apart from the occasional morepork in the distance and wind rustling in the shadows of the beech trees above.
The offending tabby and its victims
As they work, dusk changes to almost night, and something begins to stir in the nearby tree. It’s first distinguished by a sudden waft of a musky odour, not entirely unpleasant, followed by an increasing amount of scuttling inside, something like the sound of bubbling water.
The noise becomes louder, interspersed continually by high pitched squeaking. A flit of wings as the first bat emerges, a pause to see whether anything bad happened to him and (in the absence of anything bad) a sudden stream of small critters as they emerge from the huge cathedral-like entrance of the tree.
This is "The Mothership", a massive beech tree on the southern slopes of Mt Ruapehu, which can house more than 1000 short-tailed bats. A hop and a skip away another 2000 bats emerge from a tree dubbed "Notre Damme”.
Elsewhere in the forest, one or two other big roost trees will be emptying rapidly as the bats fly out to hunt. Rangers visit the roosts to set-up infrared cameras in an attempt to count the numbers as they emerge from roost trees at night. This allows them to see how the population is doing year after year.
The trees are found by catching individual bats in a net strung up in the forest, attaching a small transmitter to its back and tracking them to roost trees. As the bats tend to cycle through old roosts every year, most of the trees are already known.
Staff were shocked to visit the roosts and find small wings scattered around the trees, obviously ripped from the body. Here and there are completely intact bats, as if they’d decided to sleep outside today - the only sign of disturbance one or two puncture marks. This is the worst mass predation staff have seen, and in the first day 86 batwings are found (43 bats), the next 19 little bodies and 6 wings.
The most likely culprit was thought to be a stoat and the immediate response was to set out a trap baited with a dead bat and chicken egg. Triumphantly a big female stoat is carried out two days later. The victory is fleeting however, as the stoat’s stomach is completely empty, the pathology report indicating a predator with a much larger bite size, probably a cat or ferret, and the bats keep dying. They find 32 batwings scattered around the base of The Mothership, and 3 intact bodies and 2 wings cached nearby in a small den.
Twenty-four hours after setting out a cat trap at the base of The Mothership, Malcolm Swanney - a local DOC ranger - returns to the office with his prize and a big grin. A male tabby cat, surprisingly heavy for its small size, and although its stomach reveals no evidence of bat remains, the bite size matches exactly those found on the dead bats. The roosts continue to be watched, and as the days go by with no more wings or bodies found, and no more cats caught, the rangers finally begin to relax.
In the space of 7 days, one cat killed a total of 102 bats, and these only the ones that were found. "This really highlights the impact that cats can have on native wildlife, and I don’t think a lot of people realise what skilled predators cats really are. It’s one thing to suspect predation occurs, another thing entirely to actually witness the damage one cat can do." - Jess Scrimgeour, DOC scientist.
DOC staff from Ohakune will continue to monitor the bats and hope this was a relatively rare one-off event. For any further information about native bats, please contact your local DOC office or visit the native bats section of this site.