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Thousands of bats drop dead from trees in NSW heatwave

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Thousands of bats have dropped dead in the heat, falling from the trees as scorching summer conditions blasted south-eastern Australia over the weekend.

Among the worst affected areas are around Casino in northern NSW and Singleton in the upper Hunter, where residents and council workers say thousands of bat carcases have been discovered scattered on the ground in parks and along riverbanks.

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Sydney's short break from the heat

The city will have a chance to cool down at the start of this week before the mercury will start to rise again from Thursday onwards.

Yet more dead bats remain clinging in tree canopies, and are expected to fall to the ground as they start to decompose over the coming week, Richmond Valley Council general manager Vaughan Macdonald said.

Mr Macdonald said it became clear about 3pm on Sunday that the bat colonies were in distress, as the mercury hovered around 46 degrees in the region.

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South-eastern Australia was the hottest place on the planet on the weekend, with Ivanhoe Airport recording a maximum of 47.6 degrees and claiming the title of global hotspot.

"Once it gets beyond about 42 degrees, the bats start to struggle with the heat, they're not able to cope with it," Mr Macdonald said.

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"What happens is they literally fall out of the trees onto the ground because they're just dehydrated and they've lost all energy."

He said WIRES and the Northern Rivers Wildlife Carers representatives gathered in Casino and attempted to rehydrate some of the bats, also known as flying foxes, as they fell to the ground, while Rural Fire Service crews also sprayed water into the trees.

An estimated 2000 bats still died along the Richmond River, he said.

"They do continue to drop during the week. What happens is some literally die in the tree. They're still attached to the tree and so they gradually fall out over a period of a week," he said.

Council workers had been out in force collecting the dead bats, which can carry lyssavirus. That virus can cause a similar illness to rabies, which affects a person's central nervous system and is usually fatal.

Mr Macdonald urged people not to touch any dead bats, but to alert the council so they could be collected.

Doctor Justin Welbergen, the President of the Australasian Bat Society and a senior lecturer in animal ecology at the Hawkesbury Institute for the Environment, said the extent of the weekend's weather on bat populations was still unclear.

Initial data, however, suggested thousands of bats had died in at least seven locations on the weekend from Sydney up into south-eastern Queensland.

"Multiple flying fox camps have been affected, and thousands of flying foxes are dead. We're trying to get our heads around the exact numbers," he said.

Dr Welbergen said there had been about 25 such heat-related events along Australia's east coast since 1994, in which more than 100,000 bats died.

Black flying foxes were predominantly affected once the temperature hit 42 degrees, but other species, including the grey flying-fox, were also affected if the temperature was slightly higher.

Young bats and lactating females were particularly susceptible, he said.

"These extreme heat events have a very serious impact on the species, and those impacts then reverberate through the Australian forest ecosystems," Dr Welbergen said.

"Suddenly if you don't have any flying foxes, then there's no pollen and seed dispersal of plants that are reliant on flying foxes for these services."

Given bats live in colonies, it is relatively easy to document cases of mass deaths, as opposed to other animals that also perish in heat but live more solitary lives, he said.

"They're kind of like bio-indicators of what's happening to other wildlife in Australia," he said.

The Australasian Bat Society has produced a heat stress indicator map, predicting where colonies will be affected in heatwaves.

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