Australia is home to more hollow-dependent wildlife than almost anywhere else on earth. From tiny microbats to large owls, from sugar gliders to brushtail possums, an extraordinary diversity of native species depends on tree hollows for shelter, nesting, and breeding. But across the country — and particularly in urban and peri-urban Victoria — those hollows are disappearing faster than they can form. The result is a quiet crisis that is having a profound impact on native wildlife populations.
How tree hollows form
Natural tree hollows are not formed overnight. They develop over decades — sometimes centuries — through a slow process of branch loss, fungal decay, insect activity, and weathering. In Australian eucalypt forests, hollows typically begin to form in trees that are 100 to 300 years old, depending on species and growing conditions.
This extraordinarily long formation timeline is at the heart of the hollow crisis. Trees that were cleared for agriculture, urban development, or timber production a century ago have left a generational gap in hollow availability that will not be filled by natural processes for decades — or in many cases, centuries — to come.
The scale of the problem in Victoria
Victoria has lost the vast majority of its original native vegetation since European settlement — and with it, an enormous proportion of the hollow-bearing trees that once provided habitat across the landscape. In urban Melbourne and surrounding regions, the situation is particularly acute.
Street trees and park trees are rarely allowed to reach the age at which hollows begin to form. Trees on private property are regularly removed for development, renovation, or safety reasons before they develop hollow-forming characteristics. And where mature trees do remain, they are often isolated — disconnected from the broader landscape in ways that limit their value as wildlife habitat.
The result is a landscape where hollow-dependent species are competing for a rapidly shrinking resource — with direct consequences for breeding success, population viability, and long-term survival.
Who depends on hollows?
The range of species that depend on tree hollows in Victoria is remarkable. Hollow-dependent wildlife in the region includes:
Birds — including owls, cockatoos, parrots, rosellas, treecreepers, and many other species that nest exclusively in hollows. For large species like the Sulphur-crested Cockatoo and the Powerful Owl, large hollows in mature trees are an absolute requirement for successful breeding.
Possums and gliders — including the Common Brushtail Possum, the Common Ringtail Possum, the Sugar Glider, and the Squirrel Glider, all of which use hollows for shelter, denning, and raising young.
Microbats — Victoria is home to numerous microbat species, many of which roost in tree hollows during the day and use them as maternity sites for raising pups. Microbats are significant consumers of insects and play an important role in ecosystem balance.
For all of these species, the availability of suitable hollows is not a convenience — it is a fundamental requirement for survival and reproduction.
The impact of hollow loss
When hollow availability declines, the consequences for wildlife are direct and measurable. Species that cannot find suitable hollows experience reduced breeding success — they may attempt to nest in suboptimal locations, fail to breed entirely, or compete aggressively with other individuals for the hollows that remain.
For species already under pressure from habitat loss, predation, and climate change, hollow scarcity adds another layer of stress that can tip local populations toward decline. In urban environments, where other pressures are already significant, hollow loss can be a critical factor in the local extinction of hollow-dependent species.
What can be done?
The long-term solution to the hollow crisis is the retention and protection of mature trees — particularly those that are approaching or have reached hollow-forming age. Every mature tree that is retained in an urban or peri-urban landscape is a potential hollow-bearing tree of the future, and its value to wildlife increases with every passing decade.
Where mature trees have been lost and natural hollows are scarce, artificial hollows provide a practical and proven interim solution. Carved hollow nest boxes — made from solid salvaged hardwood to species-specific dimensions — closely replicate the conditions of natural hollows and have been shown to achieve significantly higher occupancy rates than standard manufactured nest boxes.
Installed into suitable host trees by skilled climbing arborists, carved hollows can begin providing habitat within weeks of installation — bridging the gap between hollow loss and the slow regeneration of natural hollow availability across the landscape.
A role for everyone
Addressing the hollow crisis is not solely the responsibility of governments and conservation organisations. Property owners, councils, developers, and land managers all have a role to play — through the retention of mature trees, the installation of artificial hollows, and the creation of connected habitat corridors that allow hollow-dependent species to move through the landscape.
At Shelterwood Consulting Arborist, our carved hollow nest boxes are one small but meaningful contribution to that effort — handcrafted from salvaged hardwood, installed by skilled climbing arborists, and built to provide genuine, lasting habitat for the native wildlife that shares our urban landscape.
Want to do something about the hollow crisis on your property or land? Get in touch with Shelterwood Consulting Arborist today.