Tree Hollows: Hidden Homes Worth Protecting

This National Tree Day, we’re turning our attention to something many of us walk past without a second thought — tree hollows.

They might look like ordinary holes in trees, but they’re actually essential homes for hundreds of native animals. From possums and gliders to owls, cockatoos and even frogs, more than 300 species rely on tree hollows for shelter, nesting and survival.

We spoke with Matt Stephens, a conservation biologist and founder of Hollowhog, to understand the importance of hollows, what’s causing their decline, and what can be done to protect them.

“From an ecological standpoint, how serious is the loss of tree hollows across Australia, and what impact is it having on native wildlife populations?”

The loss of tree hollows is a serious issue for many of the 300 species that use hollows as roosting and breeding sites. Across Australia around 17% of birds, 42% of mammals and 28% of reptiles call a hollow a home. Some of Australia’s most critically endangered birds and mammals rely on hollows such as Swift Parrots and Leadbeaters Possums. Hollows can take between 70 and 120 years to get to a size where they become useful for wildlife. Considering these time frames, even in forested environments, we are often trying to repair habitat damage that occurred as far back the early 1900’s. Once hollows are gone from a patch of habitat, the animals that rely on them are lost too.

“What are the main causes of hollow loss today — and are these pressures increasing or changing?”

In urban areas the main causes of hollow loss today are from continued expansion of suburbs into bushland habitats and tree removal due to the perception that larger and older trees are dangerous.

Across the rest of the country, clearing for agriculture, forestry and infrastructure are the main causes of hollow loss. It is staggering to realise that in the past 200 or so years we have cleared around 45% of the forests that once existed. If we look at more recent figures, in the past 25 years alone around 10 million hectares of native forest have been cleared. Because hollows take so long to form, even if a forest is allowed to regenerate, it will be many years before animals that rely on hollows can return.

Another recent cause of hollow loss was the devastating bushfires in 2019–20. Older trees are often more prone to collapsing during extreme fire events and these are often the trees with the most valuable range of hollows – particularly for larger species such as forest owls, cockatoos and marsupial gliders.

As our population grows, there is more pressure to clear forests for housing, roads and intensive agriculture.

There are many initiatives by conservation organisations that are resulting in the protection of habitats and in some instances, governments are taking steps to end processes such as native forest logging that has been one of the main drivers of tree hollow loss.

"National Tree Day often focuses on planting new trees — how can we balance this with the urgent need to protect old, hollow-bearing trees?"

Planting trees is a really important way to repair past forest loss and it is fantastic to see the efforts being put into their reestablishment.

Urging governments to restrict the further clearing of forests and in particular banning the removal of old growth forests is one way that we can help to protect old hollow bearing trees. Old growth forests are often the place with the highest species diversity and the old trees that contain hollows of all shapes and sizes that will support many different species. It has been estimated that an old growth forest can contain up to 150 hollows per hectare. Imagine how many animals that can support!

In urban areas we need to continue to promote the habitat value of large trees and look for ways to retain them wherever possible. Safety for people is always something that needs to be considered and by working with arborists and council’s environment teams we can often find that trees deemed too unsafe to retain can be saved. Things like under planting around a tree’s base to limit people’s access below them, careful pruning of canopies to lessen the stresses during storm events that might break limbs and other measures, are great ways to be able to retain large trees.

"What role can everyday Australians play in supporting hollow-dependent wildlife in addition to planting trees on National Tree Day?"

By far the most important thing to do is to lend a voice to efforts to protect more forest habitat. The more people that raise their voices, the more likely governments are to listen. The other thing people can do is to install a hollow or two for wildlife in their local area. This can be as simple as putting up a nest box or salvaged log hollow in their backyard or contacting their local council to urge them to add hollows to parks and urban bushland.

“Are there any innovations or conservation models — in Australia or overseas — that show promise in addressing the tree hollow crisis?”

Australia is leading the charge when it comes to ways to improve habitat for hollow using animals. For the past 30 years or so, nest boxes have been used to supplement natural hollows, with recent improvements to nest box designs that help with thermal insulation and durability. Organisations such as Wild BnB, Habitat Innovation and Thermal Haven all provide good nest box options.

An exciting recent innovation is tools and techniques to introduce hollows directly back into trees that mimic as closely as possible natural tree hollows. This includes chainsaw carved hollows and a tool that I invented called Hollowhog that allows hollows to be carved with minimal damage to a tree’s living tissue. Carved hollows have major benefits in that they can match the characteristics and properties of natural hollows and will last as long as the tree they are carved in.

The conservation effort to protect and increase the number of hollows across the landscape is still very much in its infancy. It is encouraging to see that local councils and state and federal governments are changing laws and regulations to ensure that hollows are retained and added back into the landscape. Every little bit helps and so the more people that get involved the more we can help to ensure that our children and future generations will get to see the incredible range of animals that rely on hollows.

Why Tree Hollows Matter

• Tree hollows form slowly — it takes:

  • 100 years for small hollows (for gliders)

  • 200 years for medium hollows (for parrots)

  • Over 200 years for large hollows (for owls and cockatoos)

• More than 300 Australian species depend on hollows, including:

  • 17% of native birds

  • 42% of mammals

  • 28% of reptiles
    (Source: CSIRO, PS.org.au)

• Animals that rely on hollows include:

  • Gliders, possums, quolls

  • Owls, rosellas, cockatoos, ducks

  • Bats, snakes, frogs, skinks, lizards

• Fallen logs also matter – they provide shelter for echidnas, bandicoots and reptiles

🌱 What You Can Do

This National Tree Day, take action to protect tree hollows:

• Retain dead and live hollow-bearing trees

• Leave fallen trees/logs, especially near water

• Build nest boxes suited to your local species

• Plant native trees that will form hollows in future

• Preserve native vegetation along rivers and creeks

• Speak up – call for the protection of old trees and bushland in your community