Nocturnal, Inconspicuous, Irruptive Nomads… Eastern Barn Owls

By Keith Hutton on December 13, 2011 in Other

Barn owls are usually nocturnal and roost during the day in tree hollows where they are unlikely to be seen. Occasionally they roost inconspicuously in shady trees, where they may be disturbed and fly out into the open in daylight; they are then mercilessly and noisily attacked by aggressive mobs of miners, magpies or other birds, and immediately become very obvious. They are buff coloured and pale grey in daylight but appear white when caught in car headlights after dark, sitting on a roadside fence post or floating buoyantly like a ghost just above car height, crossing the road or hunting for small mammals, head down and sometimes hovering over rough grass on the roadside.

Open country is the preferred habitat for barn owls in Australia, with scattered trees in rough grassland, on farms and around cities and country towns. Many very similar species and subspecies are found throughout the world, except in Antarctica.

Barn owls in Australia are highly selective predators that eat primarily small ground mammals about the size of a mouse, usually rodents and small marsupials, but also young rabbits, bats, birds, frogs, lizards and insects.

Eastern Barn Owls are generally uncommon in the Sydney region and their status is uncertain. There is probably a small resident population, but they are essentially irruptive nomads in Australia that gather opportunistically where plagues of native and introduced rodents occur, when they breed continuously as long as conditions are good, and then disperse when the prey numbers collapse. The rise and fall of regional populations can be dramatic and when the food supply declines high mortality and dispersion of barn owls is well documented. Up to half the population may perish under these conditions, and young owls travel great distances and may occur anywhere in Australia, in places where they have not been seen for many years, including urban areas. House mice bred in plague proportions in many areas of inland NSW in autumn this year following good rains, and recent sightings of barn owls at Manly and Maroubra may reflect dispersal as mouse numbers dropped over winter.

The introduction of house mice and development of arable farming has been of great benefit for Eastern Barn Owls in Australia and far outweighs any problems that have reduced their success in other parts of the world. For example, pesticide poisoning, limited shooting and reduction of native prey items through habitat modification are of little significance in Australia where motor vehicle collisions and natural causes probably kill most owls. Mouse plagues in Australia result in outstanding success for barn owls, which are able to capitalise on the increase in food availability through their ability to respond by continuous breeding, and to disperse widely when the good times end, despite very high mortality from starvation.