Hollow-bearing eucalypts

The yellow-tailed black cockatoo needs a much larger cavity to make a home.

xA small bird such as the striated pardalote only needs a small hollow to nest in.

It can take 80 to 100 years to form a tree hollow for smaller species of birds and animals.

The Australian wood duck is not native to Tasmania, a cavity-nesting species whose population has increased since the 1970s.

Sarah Lloyd OAM

Whether in forests, woodlands or as scattered or isolated trees on cleared land, stately old gum trees characterise the Australian landscape. 

Large old eucalypts are not only aesthetically pleasing, they are crucial for the survival of many of our native animals that use hollows for shelter, feeding, nesting, rearing young, regulating temperature and as a source of water. 

When Europeans first arrived in Tasmania, they selected the best quality stands of timber they could find to build houses for early settlers, both here in Tasmania and in the growing cities on the Australian mainland. 

Since then, numerous large old trees have been used for housing, fencing and firewood, and more recently, a more rapacious approach to timber harvesting has seen wide scale clearing and ‘conversion’ of native forests to pines and non-native eucalypts. 

Plantations are harvested before trees reach maturity and trees with hollows are now scarce in many areas. Conserving hollow-dependent fauna is an increasingly important conservation issue. 

The shortage of hollows is exacerbated by a number of factors. 

Non-native hollow users such as feral bees and starlings, the introduced laughing kookaburra, and the (probably) self-introduced little corella, galah, rainbow lorikeet and Australian wood duck have been favoured by land management practices and in many areas their large populations mean that there are few, if any, hollows left for native species. 

For example, the Australian wood duck, a grazing goose-like bird, was rare in Tasmania before the 1970s. More farm dams adjacent to rich pasture has seen an increase in these cavity-nesting ducks. 

Since British settlement, Sulphur-crested Cockatoos have also increased because wildlife parks, cattle feed lots and grain silos provide them with a ready source of grain and water which means they can breed more often.

One important factor influencing the formation of cavities is the presence, or absence, of woodpeckers. 

These efficient excavators of tree hollows occur on every continent except Antarctica and Australia. They have special shock absorbers in their heads designed to withstand the impact of drilling into tree trunks with their chisel-like bills. They can create a hollow in a matter of days or weeks. In contrast, hollows in Australia take much longer to form. 

The process of hollow formation in Australia often begins with fungal decay in the heartwood of old trees, especially those already damaged by wind or fire. The heartwood is further excavated by the activities of vertebrates and invertebrates such as white ants (termites). 

It takes 80 to 100 years to form hollows for small species, such as striated pardalotes, pygmy possums and bats. 

It takes a further 100 or more years to form cavities large enough for sulphur crested cockatoos, yellow- tailed black cockatoos, green rosellas, eastern rosellas and masked owls. 

It is essential to preserve existing old-growth trees and paddock trees. It will take many centuries before today’s revegetation creates a new home for a masked owl.