Itâ€™s dusk and, in the moments after the sun dips below the skyline, the cityâ€™s night flyers flood the sky.
In its own way, the nightly aerial show is a majestic sight.
But for many itâ€™s also a mystery. Where do these creatures come from, and where are they going?
There are thought to be about 780,000 grey-headed flying foxes in more than 100 camps dotted across Australia.
A threatened species because of dwindling habitats, they have been further battered by heatwaves over the past couple of summers that have seen them literally drop dead, by the hundreds, out of trees from Melbourne to Cairns. (Thousands of spectacled bats, also a vulnerable species, have suffered the same fate in far north Queensland.)
Flying foxes occupy the ever-shrinking forests and woodlands along the east coast, from Cairns, down through Bundaberg, Brisbane and the Sunshine Coast, into Sydney where they particularly like to hang out in Centennial Park, and in Victoria.
Melbourne’s biggest congregations are at Yarra Bend and in outer-suburban Doveton, 30 kilometres south-east of the CBD. Bats have been moved on from the lush city botanic gardens of both Melbourne and Sydney in recent years.
But none of them stay put in any one spot. These mammals are nomadic and operate in one single, mobile population, with individuals moving up and down the coastal belt in waves, taking shelter in dozens of different locations.
This means itâ€™s very possible the flying foxes you see soaring over your home after sunset could have spent the previous week in tropical Queensland or somewhere along the temperate Victorian coast.
Look up into the towering branches of flying-fox camps during the day and you will be greeted by a relatively serene sight.
Flying-fox camps are not a permanent home but are more like hostels, with a turnover thought to be roughly about 10 per cent every day.
In these parkland hostels, you will find everything from mothers caring for their newborns to travellers seeking a stopover to semi-permanent residents enjoying the local culinary scene.
The sizes of camps depend on what food is available in the area and the time of year â€“ in summer, colonies are generally larger but it’s in autumn, the breeding season, that colonies can be the loudest and smelliest.
About this time of year bat pups, born the year before, are weaned and females are ready to fall pregnant again.
Amorous males start to advertise themselves with an oil that they rub onto branches to attract the opposite sex.
â€śThe smell is musty,â€ť says Parks Victoria bat ranger Stephen Brend. â€śIf you imagine decomposing fruit, like in an orchard or fruity compost. The smell is from the male’s oils. The males are generating more oil at this time of year and it also smells more immediately after rain.â€ť
The twilight cacophony from the trees is also at its peak at this time of year. Juvenile bats, freshly weaned from their mothers, are finding their places in the colony, says Brend. â€śThey’re like rowdy teenagers on their own for the first time, on the edges of the colony, making noise.
â€śAnd then you have the mating. The males are saying, I’m here, and approaching females and females are saying yes or buzz off. In mating season, there’s more social interaction â€“ some friendly and romantic and some a little bit aggressive.â€ť
Bats have few natural predators. Further north, the odd goanna and python may be partial to a bat; in Melbourne itâ€™s more likely ravens that will try to snatch a newborn, or owls and other raptorsÂ that may try to prey on a flying fox.
So, during the day, hundreds of bats, hanging upside down, will snooze in the sunlight, attached to branches by their feet, which have a locking mechanism so they can hang effortlessly.
About 20 minutes after sunset they let go. Dropping from their branch, they use the momentum and flap their wings, which can span up to one metre and are made of soft, leathery skin.
Flying foxes take off en masse â€“ the collective nouns for bats are a â€ścloudâ€ť or a colony â€“ initially flying together to find dinner.
â€śAnyone driving down the Monash [Freeway] past Eastlink about 20 minutes after sunset will see Doveton bats flooding over the sky,â€ť says Brend of one Melbourne colony.
â€śThe same thing happens on the Eastern Freeway between the Chandler Highway and Hoddle Street, as the Yarra Bend bats fly out.
â€śThe spectacle of watching them take off, it really is amazing, they fill the sky when they leave.â€ť
In Sydney, residents of the inner-east, in suburbs such as Paddington and Randwick, witness the evening fly out.
Gradually they disperse, splitting from the pack and flying solo or in small groups.
Up in the air, they navigate using major waterways and highways to guide them.
Large, glassy eyes work as an inbuilt GPS, highly adapted for both day and night vision and particularly suited to recognising colours at night.
â€śScientists wonder if the white of some eucalypt flowers isnâ€™t an adaption to help bats find them more easily at night,â€ť says Brend.
â€śThey fly about 20km/h, and their eyesight is like a cat. We think they follow roads and rivers; they navigate with their eyes following the city.â€ť
Once they are away from the camp, the search for food begins. Itâ€™s flowers and blossoms, particularly from eucalypts, that are their first choice for a meal. In Brisbane, for example, this includes the nectar from bloodwoods,Â grey ironbark, red gum and broad-leafed paperbark, which flower at various times, providing a steady supply overall.
â€śThink of them as big nocturnal bees,â€ť says Brend, â€śpollen and nectar is what they are after.â€ť
They will also want several different types of food every night, meaning they will travel far â€“ generally about 20 kilometres but in some cases up to 40 kilometres â€“ in one evening.
If they canâ€™t get nectar or pollen, they will find fruit. And this is where they can make enemies, munching on fruit crops or orchards in their search for dinner.
Figs are big on the menu, as are mulberries; the bats pluck the fruit from the branch and tear apart big pieces using a thumb claw and foot. In this manner, they will also eat most soft-skinned fruit such as apples, peaches and pears.
To drink, they bellydive into waterways then hang upside down, allowing the water to run off their fur as they lick it up.
After a night spent flapping from one tree to the next, they return to camp just before dawn, with mothers the first to head back.
Bats creche their young, meaning when the babies get too heavy to carry after about six weeks, mothers will leave them together in the trees without supervision before returning to find their pup and take them back to the roost.
Flying foxes have been known for causing damage to commercial crops and orchards, public gardens and native vegetation since European settlement.
Itâ€™s this type of destruction that gives bats a bad name.
â€śItâ€™s really damage to commercial crops and backyard fruit trees which upset people,â€ť says Brend. He says there can also be concern about public gardens when bats start roosting in those trees, even though the damage to local vegetation is minimal.
â€śThe noise and smell of a large colony obviously has an impact. But this is why Melbourneâ€™s colonies in Yarra Bend and Doveton are so ideally situated; no immediate neighbours and no houses nearby.â€ť
Influxes of bats have been not so ideal in some other places. In Queensland, towns such as Charters Towers have had to deal with large numbers of bats at various times. Tens of thousands of flying foxes converged on Batemans Bay, on NSW’s south coast in 2016, drawn by gums flowering in the area, and bringing with them noise, droppings and power outages.
Governments and councils must delicately balance the interests of residents with the interests of bats.
The issue can be vexing not just for humans but for the hapless bats. In their hunt for tucker, for example, they can become entangled in the fine gauge netting that’s often draped loosely over fruit trees.
But itâ€™s important to remember that the vast movement of flying foxes, the nightly travels and journeys across the country, make them an integral part of the ecosystem.
They play a major role in the regeneration of native hardwood forests by long-distance pollinating as they disperse seeds during their travels. A single flying fox can scatter up to 60,000 seeds in one night.
There are perceptions that the flying fox is riddled with disease.
They are indeed hosts for Hendra virus, which can be transferred to horses by bat saliva, and then from horses to humans; and the Australian bat lyssavirus, which can be caught from untreated bites or scratches and which has symptoms like rabies.
Infection is extremely rare. Three people have died after being infected with the lyssavirus in Australia since 1996. Four people have died of Hendra virus since it was discovered in 1994, and some 90 horses have been infected in northern NSW and Queensland.
â€śVery, very few bats have lyssavirus,â€ť says Brend. â€śIf you donâ€™t touch the bats, youâ€™re safe. Itâ€™s not in their wee or poo, if the bats are carrying the virus itâ€™s in their saliva. A dog bite is more dangerous than a bat.
â€śThe biggest misconception is that they are slightly sinister creatures of the night, friends of Dracula. But in fact they are social, communicating mammals.â€ť
Flying foxes can be heard â€śgossipingâ€ť while they groom during the day, chattering like monkeys, says Brend. They have a loose social structure that sees dominant males high in a tree with up to six females around them. Males â€“ who play no part in raising the pups â€“ like to hold a territory while females seem to move around more.
In the wild, flying foxes can live for about 15 years but they are being threatened by a range of factors.
One of the biggest threats is habitat destruction; increasing conflict with people and extreme weather events are also hazards.
â€śWhat happens in Queensland will affect bat numbers and distribution in Victoria, and vice-versa. For instance, this year we are seeing a surprising number of so called ‘pop-up’ camps in Victoria â€“ for example, at Brooker Park in Warragul or Warrnambool Botanic Gardens â€“ which may be linked to the drought, fire and flood trifecta which hit Queensland in late 2018 and early 2019,â€ť says Brend.
â€śThe bats may be staying in Victoria because of a lack of food availability further north.â€ť
Heatwaves have led to mass deaths of bats.
Lawrence Pope, the president of Friends of Bats and Bushcare, says heat has the same effect on flying foxes as humans. When their core temperature rises, bats become disorientatedÂ and can collapse and die of heat stroke.
Spraying them with water can help them cool down. They also need to be separated and â€śde-clumpedâ€ť so they are not heating up in a pile.
â€śThe species is feeling the effects of climate change,â€ť he says. â€śThe forecast or climate projections show these weather events will get more and more frequent and severe. I don’t know if we can save this species. Some scientists think it will be extinct by 2050.â€ť
Stephen Brend hopes that a better understanding of bats, their contribution to the native flora and their activity around human settlements will create a kinder perception of the unique nocturnal animal.
â€śMore and more people are coming to appreciate the spectacle of the nightly fly-out and how lucky we are to share our city with this amazing animal.â€ť
With thanks to Stephen Brend from Parks Victoria.Â