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Anti-art for the birds and bats

One of my latest roost hollows for mouse-sized Sydney microbats. I's made from a hollow eucalyptus limb. The entry hole is at the bottom. Photo: Lee Hoffmann

Sydneysider: A personal journey

One of my favourite pastimes is building up Sydney’s housing stock. Not for people but for wildlife.

You’ll often hear ecologists talk about the importance of “old growth” forest. What this means is that there are lots of big, very old, and very damaged trees with a hollow trunk and plenty of hollow limbs. These are biodiversity trees. They provide nest holes for birds such as parrots, lorikeets and kookaburras, and daytime hideouts for various possums and tiny bats.

Without these essential bits of real estate, dozens of species can’t permanently inhabit what might otherwise look like a leafy suburb or even a sizeable chunk of relic Sydney bushland.

The Wolli Creek Valley in inner south-west Sydney, for example, contains some wonderful bush, but very few of its trees are more than 100 years old, which is far too young for them to have acquired more than a few small hollows.

A few years ago I decided I wasn’t going to wait around for another hundred years to pass – I’d arrange some artificial nesting places. When I was a project officer with the National Parks and Wildlife Service, I’d actually issued a leaflet with a simple design for a brushtail possum box. It was a gut-basic plywood box with a sloping roof, an entry hole and a handy length of stick attached under the hole to make access easier. The leaflet was evidently much downloaded because, occasionally, I still see my design hanging in parks and gardens.

When I decided I’d do some nest boxes for the Wolli Valley I thought it would be fun to build them so that they appeared to be a natural part of the tree.

Art draws attention to itself, but my little craft is a sort of anti-art – the point being that humans shouldn’t actually notice it, or at least recognise it as man-made, and the best pieces of my oeuvre are, I’m proud to say, pretty hard to detect. The wildlife find them quickly, however, because they’re always on the lookout for suitable real estate – which, as always with Sydney, is in short supply!

I began with some hollow sections of a slender silver-top ash I found at Batemans Bay. The tree had fallen across a forest track and a road crew had chain-sawed it into lengths about half a metre long and thrown them into the bush.

The diameter of the hollow was perfect for small parrots or lorikeets. I screwed one end of these “barrels” onto short, thick lengths of plank, as a sort of backplate. The backplates had a hanging hole at the top, with which the hollow could be hung on big galvanised nail inserted into a tree trunk.

Using a wood sculptors’ tool called an Arbortech – a metal disk with chainsaw-like teeth fitted to an angle-grinder – I carved out the back of the plank so it would fit neatly around the trunk and then sculpted the front to resemble rough eucalypt bark. Then I plugged the open front of the hollow with a wooden disk in which I’d cut out a hole just big enough for a parrot and lorikeet to pass through. A bit of staining and oiling completed the illusion.

The Wolli Valley’s NPWS field officer arranged for a tree climber and we hung the first four boxes in the bush near Girrahween Park. After that, I started producing a few more each year, experimenting with modifications and designs optimised for owls, owlet-nightjars, kookaburras, sacred kingfishers and parrots.

A couple of years ago I was asked by Marrickville Council’s urban biodiversity people to produce something suitable for mouse-sized microbats, which since the revegetation of the Cooks River parklands with native trees, have begun to establish small populations.

Few people ever notice them, but microbats are a wonderful part of our fauna. Australia-wide, there are about 50 species, with ten or more found in Sydney. Microbats zoom around in the night, navigating with sonar and eating huge quantities of flying insects. During the day they hang out in tree hollows, caves, old mine shafts and deep recesses in old, disused buildings. When I was doing tours for the NPWS, we used to find them in the old underground emplacements of Sydney Harbour’s historic fortifications.

Microbats can use the sort of tree hollows favoured by birds and possums, but with these at a premium, they’d soon get displaced. The trick is to build them something that’s well-suited to their needs and pretty much useless for anything else. So we’re trialling vertical hollows, with the top sealed and the entry hole at the bottom. The little bats clamber up into this dark, secure, tube-like space and roost by hanging upside down from the rough interior surface. The easiest way to disguise these artificial roosts is as a length of dead wood that’s snapped off the top of a tall tree and got caught up lower down in the tree (park rangers call them “hangers”).

The first of these artificial bat-homes are now up and it just remains to see if the prospective tenants like them.

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