Conservation management and diets of Powerful Owls (Ninox strenua) in outer urban Melbourne, Australia

The Powerful Owl Ninox strenua is Australia’s largest owl, and is mainly found east of the Great Dividing Range on the mainland in tall-open forests. The species is considered rare, both nationally and in the State of Victoria; and threatened in the Greater Melbourne area. Recovery plans for the future conservation management of N. strenua are being prepared in 2 states.

Historically, Powerful Owls have been thought to require large homes ranges (about 1000 ha per pair) in suitable old-growth forest, which provides nest hollows for the owls and their arboreal marsupial prey. Recent research, however, has found N. strenua may be more numerous and breed more successfully in a wider range of habitats than previously believed. In particular, the birds have been found living in forests and woodlands within the greater metropolitan areas of cities. The most extreme case is where a nest tree has been found within 800m of urban settlement and 6km from the centre of Brisbane.

In this paper we report on the diet, habitat use, and conservation management by a number of breeding pairs of owls in outer urban Melbourne. Study sites range from a relatively undisturbed rainforest habitat 80km from central Melbourne, through dry sclerophyll, eucalyptus-dominated open forest with some disturbance to a site 8km from central Melbourne in highly disturbed urban parkland.

Diets of the families of owls were determined by analyzing remains in regurgitated pellets. The data confirm that arboreal marsupials constitute the major prey items, especially the Common Ringtail Possum Pseudocheirus peregrinus. There were differences in diets depending on the availability of prey species, which suggest a level of opportunism not previously suspected. Our study is also the first to confirm the owls capture adult Common Brushtail Possums Trichosurus vulpecula (15% of pellets containing the remains of this large opossum have bones of mature adults at 1 site) and thus take prey up to two and a half times their own weight. As well our data suggest Powerful Owls are not restricted to hollow-dwelling prey, as in some sites the marsupials rested during the day either in leafy nests called dreys (P. peregrinus) or in house roofs (T. vulpecula).

In the most heavily disturbed sites, breeding success has been reduced, and we have evidence that in one particular year the young were eaten by one of the parents. This followed construction of a bicycle track under the nest during the breeding season. Recommendations are made for the future conservation and habitat management of Powerful Owls in the Yarra Valley corridor.