Hollow homes

Monitoring our reserves in north-west Victoria, Dja Dja Wurrung Country, helps future homes for hollow-dependent fauna.

Once sweeping across three million hectares, Victoria’s Box-Ironbark forests and woodlands are a rush of blue-green eucalypts and home to many critically endangered native fauna. But since colonisation began, the ecosystem has suffered significantly from land clearing associated with agriculture. In the north-west of the state, just 17% of original vegetation remains. Within remnant pockets – particularly in gums and boxes – large, deep hollows form as trees lose their branches. These can take centuries to shape and offer vital shelter for a diversity of feathery and furry animals. 

“For Australian species, hollows are everything,” says Caleb Traher, an ecologist for Bush Heritage Australia who completed an internship through the Seeding the Future program.

“Without them, many animals can’t breed or reproduce. They can’t hide from the elements or predators,” he says. “If we don’t have hollows, then we won’t have Greater Gliders (Petauroides volans), Leadbeater’s Possums (Gymnobelideus leadbeateri), Owlet-nightjars (Aegotheles cristatus), Barking Owls (Ninox connivens), or Phascogales (Phascogale tapoatafa).” 

Caleb recently undertook research on Bush Heritage’s Daylenong reserves, which are located on Dja Dja Wurrung Country near Bendigo in north-west Victoria. The reserves protect a stronghold of more than 700 hectares of Box-Ironbark forests and other vegetation ecosystems, his research aimed to determine how many trees support hollows and which hollow-dependent species call the reserves home. 

Over the course of six months, he visited 35 sites in north-west Victoria – setting up camp at each one just before sunset. After night fall, monitoring began. Caleb used a combination of three different methods: passive listening, call playback with integrated thermal imaging, and spotlighting. For the call playback component, he used a speaker with four target species loaded onto it. 

“We had Boobook Owls (Ninox novaeseelandiae), Barking Owls, Owlet-nightjar and Powerful Owls (Ninox strenua). We’d play their calls. Wait ten minutes. Play their calls. And repeat,” he says. While the call playback was in operation, Caleb would use a thermal scope to scan for movement. 

“The second spotlighting component was pretty straightforward,” says Caleb. “Chuck on a head torch and walk through the two-hectare transect … looking for eye shine or any movement.” But Caleb also added an extra component at some sites: acoustic monitoring for microbats.

Using a recorder strapped to a tree, he detected six species of microbats. Caleb also developed a tool, dubbed ‘BatGPT’, that uses Large Language Models to help interpret echolocation calls. Caleb says gathering microbat data could prove crucial for future conservation efforts. 

“White Nose Syndrome is anticipated to move from Europe to Australia within the next 10 years, and that’s likely to be very devastating for our microbat population,” he warns. White Nose Syndrome is a fungus, Pseudogymnoascus destructans, which grows on bat’s skin during hibernation. The fungus disrupts the bats’ winter downtime, depleting the energy they need to reserve for the warmer months. The disease erodes their wing membranes and impacts flight. Overseas, the fungus has led to mass mortalities. 

While the microbats appeared to be safe for the moment, out of the other 23 species Caleb monitored, some were noticeably absent, such as Brush-tailed Phascogales and Barking Owls. 

“No one had ever seen them, but they’re both cryptic species that wouldn’t show up unless you did a comprehensive project like this,” he says. “When we didn’t find them on the reserves, we started looking at why and proposed that Barking Owls need a really high density of large, 100-year-old hollows.” 

Caleb used tree diameter modelling data to determine which trees on the reserves are large enough to sustain hollows and found only three sites contained suitable trees. He then made precise recommendations on habitat restoration efforts, such as the installation of artificial hollows, where to place them, how many and their size. 

“If we can encourage a couple of breeding pairs of Barking Owls back, that will be a huge win for these ecosystems,” says Caleb •

We gratefully acknowledge the visionary cohort of philanthropists who support the Seeding the Future Program, with special thanks to the Vincent Fairfax Family Foundation who provided substantial funding for the pilot program.