Nesting Hollows - Birds Australia

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Nesting Hollows

There are a surprising number of Australian bird species that rely on hollows for nesting sites. Cockatoos only nest in tree-hollows, and most other species of parrots do too; so do most owls. In many regions there is a shortage of available hollows suitable for building nests in. In some areas these shortages are the result of clearing. Hollows only form in the trunks or branches of old trees. When forests or woodlands are cleared, or even when they are selectively logged, the oldest and largest trees are usually the first to go. This immediately robs hollow-nesting species of nest sites; in the case of the Powerful Owl, many of its preferred mammalian prey species, such as possums, gliders and phascogales, also live in tree-hollows, compounding the adverse effect.

In areas where clearing of old trees has occurred, sometimes only a few hollows remain, meaning that there is much competition for breeding space. Some species are very aggressive when defending nest hollows, and less aggressive species are left by the wayside. The expansion of agricultural land in some areas has allowed the populations of some species, such as Galahs, to explode and become invasive species, expanding their range into areas where they were previously not present. In these recently colonized areas, these newcomers may compete for nest hollows with other species such as Carnaby's Black-Cockatoo in areas where the clearing of native woodland has resulted in few tree hollows remaining. The pressure from this increased competition for nest-sites is a major threat to the species. Similarly, some invasive exotic species, especially Common Mynas and Common Starlings, are especially aggressive around potential nest-hollows, and may even physically evict the nests, eggs or young of native species from the hollow to build their own nest, and chase the original owners away.

Competition for tree hollows does not only stem from other birds. On Kangaroo Island in South Australia, the Glossy Black-Cockatoo is threatened by competition for hollows with Common Brushtail Possums, which are also significant predators of the Cockatoos' eggs. Hollows are also often taken over by swarms of feral honeybees to build their hives.