The site is one of the few maternity colonies left in Sydney and is home to about 20,000 flying foxes.

At the moment the pregnant mothers have nabbed the best spots in tree hollows, leaving the younger mammals to skylark their way through the daylight hours.

''They are really intelligent and it's great to stand and see what they get up to,'' said Fairfield Council's environmental officer Megan Haberley.

The council and the Department of Environment and Climate Change will host an open day at the reserve on Saturday to show people how they can help protect the endangered grey headed flying fox.

Over the past five years bats at the Cabramatta reserve have been battered by heatwaves, which killed 3000 of them in 2003, and have had their habitat threatened by ibises.

The birds' droppings are toxic to bats and they destroy ground cover, leaving nowhere for the flying foxes to escape the heat.

''After the [ibis] control program they didn't come back for a year to breed and then last year only a small number returned,'' Ms Haberley said.

''We're just going to keep an eye on them because they are native animals as well and at the moment the numbers aren't high enough to be a problem.''

As for heat stress, the Cabramatta Creek Flying Fox Committee will collect any animals that have dropped out of the trees and cover them with wet towels and force-feed them water.

Ms Haberley said it was important to report dead or injured bats to WIRES and Sydney Wildlife because they could be carrying babies.

A Threatened Species Demonstration Site open day will be at the Cabramatta Creek Flying Fox Reserve, Liverpool Street, Lansvale (via the Sunnybrook Hotel car park) on Saturday, 10.30am-2pm.