The nightly fly-out at Wee Jasper begins with a faintly audible whirring flutter, like a butterfly's wing beat amped-up by a powerful microphone.
That's the sound of the first few eastern bent-wing bats leaving a hillside nursery cave - little more than cramped crawl-space - in the Goodradigbee Valley north-west of Canberra.
These are micro-bats, chocolate-coloured creatures roughly the size of a mouse, and it takes the practised eye of NSW Parks and Wildlife Service senior scientist Doug Mills to point out the fast-moving winged shapes circling in the twilight sky.
But in minutes, the faint flutter has built to an eerie pulse of sound - thousands of wing beats echoing within the limestone tunnel under the hillside - as 18,000 female bats soar out into the night sky to forage for insects. The sky is full of the tiny creatures whirring past at speeds of up to 30km/h as they head for nearby bush and farmland in search of moths and beetles.
''At peak rate, you'll get around 600 bats a minute flying out,'' Dr Mills says.
''The fly-out pattern depends on the weather. If it's a cold night, there's more urgency so somehow, even deep within the cave, the bats can predict the night's weather. We think they probably react to barometric pressure.''
The Wee Jasper cave roost (hard to spot among a hillside scatter of rocks and blackberries) is one of only three maternal colonies in NSW for this threatened micro-bat species.
The females congregate here for six weeks over summer to give birth to pups, leaving in autumn to disperse across the landscape.
Banded bats from this colony have been found as far afield as Eden on the NSW South Coast, and Gabo Island, off the East Gippsland coast in Victoria. They can fly up to 30km a night to a foraging site, and eat half their body weight in insects each night.
''They're a long-lived species, and can live up to 30 years. There's a definite social hierarchy to a colony, and we have information that suggests bats go to the same location every night to feed, and even fly out of the cave each night in the same order,'' he says.
Dr Mills is using infra-red video cameras, and computer software developed by United States military laboratories, to map bat numbers and flight paths.
It's the only study of its kind in Australia, and the techniques developed by Dr Mills to adapt the software to thermal tracking are now being used to monitor bat colonies in South Australia.
Wildlife monitoring surveys can be costly and labour intensive, but this portable, minimal cost method can, literally overnight, deliver a bat population count, map fly-out patterns, and provide data on the colony's reaction to disturbances such as intrusive noise or light.
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