The powerful owl

By day the eucalypt forest is loud enough. Kookaburras detonate from dead branches, lorikeets tear shrieking through the blossom, magpies carol. But the real business of the place keeps its own hours, and the powerful owl works the top of the night shift. Ninox strenua is the largest owl on the continent, and after dark it hunts the canopy as nothing else here can, plucking gliders and possums clean off the branches. Greater glider, ringtail, sugar and squirrel glider — all taken out of the dark mid-clamber. A pair needs a wide sweep of old forest to find enough of them, and their low, carrying double hoot at dusk is one of the defining sounds of a gum forest still working.

The owl is worth reading because it leans on that forest twice over, and both dependencies bite.

To breed, it needs a hollow, and not any hollow. It needs a very large one, a deep vertical spout in a big old tree. This is the century-old apartment the whole forest queues for. Being the biggest owl, the powerful owl needs among the biggest hollows — the rarest, the slowest to form, and the first to go under the saw. In the wet and dry sclerophyll of South-East Queensland, chambers large enough for fauna barely begin before a tree is a century old, and the truly big ones wait on trees of two hundred and fifty years and more. That is the owl’s nursery, and nothing makes one fast.

But the hollow is only half of it. An apex predator also needs prey, and a great deal of it. No quantity of hollows will feed an owl in a forest emptied of gliders and possums. Here the dependence deepens, because the owl’s prey are themselves hollow-dwellers, each glider and ringtail tied to its own old-tree den and its own spread of feed trees. So a breeding pair rests on the whole system standing intact at once: the century-old nest tree, and a large territory of forest still stocked with arboreal mammals, which in turn means their hollows and feed trees are holding too. The powerful owl does not sit lightly on the forest. It sits on all of it.

Which is why a resident pair is worth so much as a reading. Perched at the top of the web and leaning on everything beneath it, a breeding powerful owl certifies that a great deal underneath is still working: enough old hollow-bearing trees to nest in, and a prey base wide and rich enough to hunt across. Pull out the old trees and the owl loses its nursery. Thin the forest until the gliders fade and it loses its larder. Either way the top of the web comes down, and it comes down quietly. A stretch of forest can look green and sound busy by day, full of lorikeets and carolling magpies, long after it can no longer hold an owl.

So read the low double hoot at dusk as one of the surest signs that a stretch of gum forest is still whole, canopy to hollow.