Grey-headed Flying-foxes in South Australia
Extreme heat events
BE AMAZED BUT KEEP YOUR DISTANCE
Heatwaves can be a real struggle for some native animals and Adelaide’s Grey-headed Flying-foxes (Pteropus poliocephalus) are no exception.
If you visit parts of the River Torrens during hot weather you may see this wildlife spectacle of flying-foxes dipping their bellies in the river. Bats then land in nearby trees to lick the thirst quenching water from their wet belly fur. Drinking during the day is not typical and shows just how heat stressed they are. You may also see constant flapping which is an attempt for bats to keep cool, but it can tire and dehydrate them, often forcing them to drink or perish.
Unfortunately it's possible that young flying foxes will die in heatwaves. If you find a dead or distressed bat on the ground, phone Fauna Rescue SA’s bat helpline on 0475 132 093. Having only experienced and vaccinated people handle bats means that people remain safe and bats are appropriately cared for.
If you are along the Torrens there are a few important things to keep yourself and bats safe:
Don’t touch bats that come to the ground.
Try not to get close to trees that are full of bats. They may fall out and are more likely to be stressed by people being nearby, particularly if the bats are resting on branches close to the ground.
Try not to make too much noise. In hot weather bats often rest in alternative trees close to the water where they are more sensitive to disturbance than in their normal camp site.
Keep your distance from any bats that have fallen to the ground or into the river. Their response will be to climb to safety in the nearest tree, so please don’t block their path.
Bats drinking in the river are approaching their temperature thresholds and are typically very tired. This reduces their responsiveness and increases their chances of making flight errors and accidental collisions. Reduce the chance of a collision by sitting down if you are watching the bats.
When jogging or riding in the area during hot weather, keep to more elevated paths away from the river if possible.
Refrain from taking dogs into the area as this puts additional stress on the bats and poses a high risk of bat-dog interactions.
Incidences of extreme climate events are increasing in intensity, duration and frequency. Extreme heat events are extended periods of several days or more, of very hot temperatures (>40ºC).
Roosting among exposed branches in tree canopies makes flying foxes particularly sensitive to extreme heat. This heat can cause widespread mortality to flying foxes across their range.
High summer temperatures, which coincide with low humidity, occur around the same time as pups are being raised. As pups are often less capable of moving to more shaded or cooler locations, these young individuals have been most prone to heat mortality in South Australia.
During extreme heat days flying foxes will fan their wings, seek shade, pant and spread saliva over the body. If heat stress persists, bats start falling out of the trees and crawling around on the ground looking for cool areas. However, bats are close to death when they fall from their trees. Understorey vegetation that includes shrubs would normally provide cool refuges beneath roost trees in natural areas, but these habitats are often absent from urban camp sites.
Fauna Rescue monitors Adelaide’s Grey-headed Flying-fox camp during extreme heat conditions. Their actions reduce the likelihood of unvaccinated people coming into contact with bats and enable rehabilitation activities for affected individuals.
Arrival and establishment
Grey-headed Flying-foxes have been recorded intermittently in South Australia for many decades, with a permanent camp only becoming established in Adelaide from 2010.
Since 2010, Grey-headed Flying-fox numbers increased substantially, with a peak of around 22,000 in 2018, and down to 17,000 in January 2019. See the graph below. The 2019 decline appears to be due to a seasonal reduction in available food, which has forced a proportion of the colony to move interstate.
The arrival of these bats in Adelaide corresponded with other large movements of grey-headed flying foxes at the same time (in 2010). It is expected that drought conditions in parts of the species’ range forced animals to look further afield for food, with reports of the species in other unexpected areas such as Tasmania and islands in Bass Strait.
In the last 30 years, the flying foxes’ range has been contracting in Queensland and New South Wales and expanding southwards into Victoria and South Australia. Drought conditions are also likely to be compounded by existing pressures relating to habitat loss and the effects of climate change.
Historically, South Australia would have been inhospitable to these bats, however, urbanisation has created new habitats and opportunities for them to access year-round water and food supplies, including from native and non-native urban tree plantings. The reliability of these urban food resources may also reduce the need for migratory movements.
As occurred in Melbourne 30 years ago, the establishment of a permanent grey-headed flying fox camp in Adelaide represents a response by a native species to the processes of urbanisation.
With Adelaide receiving the hottest and driest conditions within their national range, it seems unlikely that the species will venture further west in South Australia where conditions get progressively hotter and drier.
See also Future trends for flying foxes in South Australia.
Bat camps and roosting ecology
Flying foxes hang upside down in trees to roost during the day, usually with their wings folded or wrapped around their bodies.
They are vocal, intelligent and social animals, aggregating in large numbers at their roosts. These “camps” are important sites for social organisation, and provide protection from predators. Camp sites can be used regularly for long periods (e.g. 100 years+).
Flying foxes may use a range of habitats for their roosts but typically they set up camps in tall, reasonably dense trees adjacent to a water source. In Adelaide for example, Grey-headed Flying-foxes roost in tall trees near the entrance to the Adelaide Zoo, beside the River Torrens.
Grey-headed Flying-foxes can tolerate some level of human disturbance. These urban camps provide the bats with easy access to food and water within their preferred foraging range of 20 kilometres.
Bats have camp sites that they use permanently or intermittently, and tree health can decline at continually-used sites. Adelaide has only a single camp site where bats roost continuously and tree health is being monitored.
It is not uncommon for Grey-headed Flying-fox camps to comprise tens of thousands of individuals. As a result, camps can be raucous with distinct smells. Males will use loud calls and strongly-scented secretions to mark their mating territories. Females also vocalise to find their young within the camp.
Listen to an audio file of the Adelaide bat camp.
Bats leave the camp around dusk each evening and return before dawn. Departure time may fluctuate with food availability; bats may leave earlier if food is limited or far away and later if food is plentiful or nearby.
Foraging range of Grey-headed Flying-foxes from the Adelaide camp site; optimal foraging range is within 20 km of the camp site (inner circle), but bats may range up to 50 km from the camp (outer circle) each night in search of food.
In South Australia, Grey-headed Flying-foxes feed in tree canopies on blossom and nectar of banksias, grevilleas, tea-trees and gum trees e.g. spotted and lemon-scented gums, and on the fleshy fruit of date palms, lilly pillys, Moreton Bay figs, and mulberries.
Future trends and expectations
The Grey-headed Flying-fox colony in Adelaide increased steadily from 2010. However, increased numbers were primarily from movements of bats into the area from interstate, and not from breeding. Successful breeding in SA has been intermittent, with high temperatures impacting on the survival of young most years.
The colony growth is anticipated to stabilise when food resources become limiting. When this happens, seasonal increases and decreases in food availability are likely to see a corresponding change in the size of Adelaide’s flying fox colony. For example, one permanent camp in Melbourne can increase from 8000 in winter, to more than 30,000 in summer. It appears reasonable to expect that 20,000-30,000 animals will eventually reside at the Adelaide camp site during times of peak resource availability.
The recent (December 2018) decline in numbers suggests that we may have reached carrying capacity, expressed as a shortage of food. The notion of a lack of food has also been supported by some animals captured, during a recent University of Western Sydney research exercise (in 2018), which saw animals in poorer condition than previously seen in SA, and juveniles coming into care being underweight for their size.