Tree Hollows

Most of us take them for granted, but what do we really know about how the tree hollows we see are used?

How many birds actually use tree hollows?

Some 114 species of birds, or 15% of birds, use tree hollows for nesting. 46 of these are already listed as threatened species. The hollow has to be right for the particularly species too, with entrance size being particularly important to birds, smaller is better, to keep predators at bay. As well as birds, another few hundred other vertebrates use hollows, so they are extremely important.

This pair of Australian king parrots (Alisterus scapularis), checked out this hollow in our garden. It’s near the start of the creek walk at the entrance end. The male is the brighter red in the second photo, and the female has more green. Many thanks to Ellany Whelan for these photos.

How long does it take for a hollow to form?

Hollows are created in a number of ways, insects, such as termites, help to create them and fungal decay is another major factor, so the process is extremely slow, it can take 120 years before cavities even start to form. Lightning strikes can create breaks that form hollows too.

Of course we should consider our own powerful owls, who have successfully nested in the garden on several occasions. A powerful owl needs a hollow that is at least half a metre deep. They generally choose eucalypts, and to have an appropriate size hollow they must be at least 150 years old. This needs to be considered when older trees are removed. Even if we do plant two or three to replace them, what do the owls do in the interim 150 years? This chap knows he wouldn’t survive.

Why are we losing so many hollows?

Timber harvesting is a major issue, especially in old growth forests. Though the loggers argue that they rotate through forests only every 50-70 years, it can take much longer than that for a hollow to form eg a tallowwood tree needs 170-200 years to form a hollow, a Scribbly gum 200-235 years. Trees that are left often die due to root damage by heavy machinery.

Housing development that clears vast areas.

Non-native tree species like camphor laurel and privet that are fast growing and push native species out. These invasive species are also masters of reproduction with their seeds travelling down rivers and creeks and creating havoc where ever they go.

Trees are often also dying earlier, even before they can form hollows, because of root damage by urbanisation eg paths and drains.

Firewood collection can also be an issue with people felling dead trees that are still perfectly good for birds and other animals to nest in and also removing fallen timber that acts as homes and breeding areas for food species.

Finally hunters. Yes we have hunters who target nest boxes and hollows.

This loss of hollows also increases the competition for hollows, with marsupials like possums and gliders also using hollows and more aggressive birds pushing out quieter birds. We also need to remember that it isn’t just the tree with the hollow that is needed. The surrounding area needs to provide shelter for the food sources of nesting birds.

These rainbow lorikeets (Trichoglossus haematodus), are busy investigating this hollow.

What are the alternatives?

Creating nesting boxes can help some species, though those needing much deeper holes are difficult to cater for. Here is a site that offers “how to” instructions if you would like to try it out. Backyard Buddies. A quick google will bring up more options and there are even people who will make them for you. We have even put up a few within the garden to help out. You’ll find this one on the nature walk.

If you have a bush block you can do several things that will help:
– retain live and dead hollow bearing trees.

– protect all old growth forest sections

– plant local, native, hollow-producing species.

These scaly breasted lorikeets (Trichoglossus chlorolepidotus), found this tiny hollow that most humans wouldn’t even see.

Many thanks to Laurence Sanders for the following photos of birds using hollows. Two super shots of Australian owlet nightjars (Aegotheles cristatus), enjoying a little sun at the opening of their hollows, an eastern barn owl (Tyto alba), a dollarbird (Eurystomus orientalis) and a tree martin (Hirundo nigricans).

Photographing birds in hollows is pretty difficult work, and you also have to be very careful not to disturb nesting birds so thanks too to Paul Van Gaal who extended our photo range with these beautiful additions of a spotted pardalote (Pardalotus quadragintus), sulphur crested cockatoo (Cacatua galerita), a southern boobook (Ninox novaeseelandiae) and a powerful owl chick (Ninox strenua).

This hole was much lower down the tree than many and we spotted an antechinus using it as a nesting hole, (too fast for our cameras), so it’s not all about birds.

It is important that everyone support organisations that support the retention of old growth forests and other groups who are trying to help. Organisations like the Australian Museum have projects dedicated to tree hollows. Hollows as Homes is a Sydney project.

This galah emerges from his nesting hole.

Want to know more?

The Brisbane episode of the ABC series “The Secret Lives of Urban Birds” has a good segment on tree hollows. You can see it on ABC iView.

“Tree Hollows and wildlife conservation in Australia” Gibbons and Lindenmayer. CSIRO Publishing, gets into the science and details hollow depths required and more about other mammal’s use of hollows.

The bottom line.
We have over 300 hollow users in Australia, of which over 100 are endangered. Generally trees need to be 120-180 years old for large hollows. Let’s all do what we can to preserve existing, and create new, hollows.

Article and photos by Cheryl Cooper unless otherwise credited. Special thanks to Ellany Whelan, Laurence Sanders and Paul Van Gaal for the use of their photos.