The powerful owl (Ninox strenua), a species of owl native to south-eastern and eastern Australia, is the largest owl on the continent. It is found in coastal areas and in the Great Dividing Range, rarely more than inland. The IUCN Red List of Threatened Species also refers to this species as the powerful boobook.
An apex predator in its narrow distribution, powerful owls are often opportunists, like most predators, but generally are dedicated to hunting arboreal mammals, in particular small to medium-sized marsupials. Such prey can comprise about three-quarters of their diet. Generally, this species lives in primary forests with tall, native trees, but can show some habitat flexibility when not nesting.
The powerful owl is a typically territorial raptorial bird that maintains a large home range and has long intervals between egg-laying and hatching of clutches. Also, like many types of raptorial birds, they must survive a long stretch to independence in young owls after fledging. Unlike most raptorial birds, however, male powerful owls are larger and stronger than females, so the male takes the dominant position in the mating pair, which extends to food distribution.Hume, R. (1991). Owls of the World. Running Press, Philadelphia, PA.
Description
The powerful owl has a long tail and a small head, giving it an atypical silhouette for an owl and imparting a more hawk-like appearance than any other large owl. The protruding bill and distinct brow ridges enhance the hawk-like appearance of the species. The facial disc is ill-defined. The upper parts are dark grey-brown, mottled, and barred with whitish. The underparts are white with bold, grey-brown, V-shaped barring. The tail has six narrow white bars contrasting with grey-brown. This species has large yellow eyes, with greyish feathering down to the base of the toes and feet of a dull yellow colour. They are aptly named, with very powerful and heavy claws. This owl is the largest species of the "hawk owl" group found in much of Asia and the Australasian region, all included in the genus Ninox. It can be considered, along with its sister species the rufous owl (N. rufa), as Australia's analogue to the genus Bubo.
This species measures in length and spans across the wings. Unlike in a vast majority of owl species, the male is slightly larger than the female on average. Body mass in males has been reported at , with 13 males averaging , while females can weigh from , with an average in 9 females of .Hollands, David (2008) Owls, Frogmouths and Nightjars of Australia, Bloomings Books, p318Dunning, J. B. Jr. (2008) CRC Handbook of Avian Body Masses CRC Press (2nd Edn) . Among all the owls in the world, the powerful owl is the ninth longest from bill-to-tail, the tenth heaviest and the eighth longest winged.Mikkola, H. (2012) Owls of the World: A Photographic Guide Firefly Books Its body mass is about the same on average as the great horned owl (Bubo virginianus), but it has a proportionately much longer tail and wings than that species. The wing chord length further illustrates slight sexual dimorphism in favor of the male, with the male measuring and the female measuring . The tail can be or more in length, the fifth longest of any owl with only the great grey (Strix nebulosa) and ural owls (Strix uralensis) having proportionately longer tails. As a relatively geographically restricted species, there are no subspecies and no known geographic variation in the powerful owl.Weick, F. (2007). Owls (Strigiformes): Annotated and Illustrated Checklist. Springer. In comparison to the barking owl (Ninox connivens), it is much larger and lacks that species' white-spotted smoky-brown colouration on the upperparts and dark grey to rusty streaks on a whitish background on the underside. The other larger and relatively powerful owl found in Australia, the rufous owl, is not found in the same range in the wild so poses no identification problem.
Voice The male powerful owl's song is an impressive low, rather mournful-sounding and far-carrying double-hoot, whoo-hooo, each note lasting a few seconds at least, broken up by a brief silence and the second note being usually higher pitched than the first. The female has a similar call, but has a higher-pitched voice. Duets are frequently heard at the onset of breeding. Unpaired males frequently call much more regularly than paired ones.
Distribution and habitat
Their range is from Eungella and the Dawson River in Queensland south to the central highlands of Victoria and west to Mount Burr in South Australia, with the range terminating around Portland, Victoria. The habitat of the powerful owl is tall, humid forests ranging through to some drier woodlands in northern Victoria and the western slopes of New South Wales and Queensland. They can be found in wooded mountain gullies, forested ravines, wetter, heavily timbered subcoastal ranges, coastal forests and woodland, and coastal scrub. They prefer wetter, more timbered areas such as sclerophyll forests. Although usually associated with subcoastal forest, they can be found on inland mountain slopes occasionally. This is especially the case with young owls after dispersal before they can establish their own breeding territories. Recent mapping work has shown that streams between ridges covered with Eucalyptus forest are often prime habitat for this species. They also occasionally range into plantations, mainly of pine or native tree species, and urban and rural parks and gardens.
Behaviour
The powerful owl lives permanently in breeding pairs once in adulthood. They have been observed roosting singly, in pairs, and in family groups of three or four. They frequently roost during the day on branches in tall, open trees, often while still holding the prey caught the prior night. Several perches are used, and may be occupied intermittently for years at a time. The powerful owl typically flies in a slow and deliberate way on its large wings.
Diet
The powerful owl is the top nocturnal predator of the forests and woodlands in its range. About 75% of the diet of the powerful owl is made up of arboreal mammals. Its diet consists largely of arboreal marsupials such as flying foxes (Pteropus spp.), the southern greater glider (Petauroides volans), ringtail possums (Pseudocheiridae), brushtail possums (Trichosurus spp.), koala (Phascolarctos cinereus), sugar glider (Petaurus breviceps), and feathertail gliders (Acrobates pygmaeus).Seebeck, J. H. (1976). "The diet of the powerful owl Ninox strenua in western Victoria." Emu, 76(4): 167–170.Cooke, R., Wallis, R., Hogan, F., White, J., & Webster, A. (2006). "The diet of powerful owls (Ninox strenua) and prey availability in a continuum of habitats from disturbed urban fringe to protected forest environments in south-eastern Australia." Wildlife Research, 33(3), 199–206.König, Claus, Friedhelm Weick, and Jan-Hendrik Becking. Owls of the World. 2nd ed. Bloomsbury Publishing, 2009. Web. 23 May. 2023. These prey species vary extensively in body size, from the feathertail glider, which has an adult weight merely of to the koala, typically weighing , though juveniles are mainly taken in large prey species.Kavanagh, R. P. (2002). "Comparative diets of the powerful owl (Ninox strenua), sooty owl (Tyto tenebricosa) and masked owl (Tyto novaehollandiae) in southeastern Australia." Ecology and Conservation of Owls, pp. 175–191. The average estimated prey weight per one study was approximately .Olsen, J., Judge, D., Trost, S., Rose, A. B., Flowers, G., McAuliffe, J., ... & Maconachie, M. (2011). "Powerful owl Ninox strenua diet from two sites in the Australian Capital Territory." Australian Field Ornithology, 28(3): 120. However, another two studies stated the mean prey weight as and . Not infrequently, prey weight averages between 50 and 100%, whereas, in most other raptors, including large owls, there seems to be a "rule" that most prey weighs 20% or less of the raptors' own weight. The mean weight of prey for powerful owls can be up to 10 times greater relative to their body weight than the mean prey weight of similarly sized northern owls, such as great grey or great horned owls.Olsen, Jerry. (2011) Australian High Country Owls. CSIRO Publishing
The most largely taken prey species in many regions are adults of common ringtail possum (Pseudocheirus peregrinus) which can weigh around .Cooke, R., and Wallis, R. (2004). "Conservation management and diets of powerful owls (Ninox strenua) in outer urban Melbourne, Australia." In Proceedings of the 4th International Symposium on Urban Wildlife Conservation, Shaw, W., Harris, L., and Vandruff, L. (Eds) pp. 110–113. Larger common brushtail possums (Trichosurus vulpecula) of all ages can be taken, but young specimens are preferred since they are abundant in spring and easier to catch than adults around in weight. On some occasions, they even take both young and adults of mountain brushtail possums (Trichosurus cunninghami) which can grow about . Aside from possums, smaller sugar gliders, weighing and much larger greater glider, which are the same size of ringtail possums, can also be the most prominent prey species in the powerful owl's diet in many regions.Kavanagh, R. P. (1988). "The impact of predation by the powerful owl, Ninox strenua, on a population of the greater glider, Petauroides volans." Australian Journal of Ecology, 13(4): 445–450.
Not infrequently taken are the black (Pteropus alecto) and grey-headed flying foxes (Pteropus poliocephalus), the largest of Australian bats, although smaller bats have also been killed. Introduced mammals, namely rats (Rattus spp.), European hare (Lepus europaeus), and European rabbits (Oryctolagus cuniculus), especially the abundant young of the latter after their litters disperse, are also hunted. Opportunistic as are most predators and owls, given the chance the powerful owl will also prey on nocturnal birds such as the tawny frogmouth (Podargus strigoides). Roosting diurnal birds are also taken such as various cockatoos and parrots (around a dozen species thus far recorded), Australian brushturkey (Alectura lathami), dusky moorhen (Gallinula tenebrosa), white-faced herons (Egretta novaehollandiae), crested pigeon (Ocyphaps lophotes), wonga pigeon (Leucosarcia melanoleuca), kookaburras (Dacelo spp.), superb lyrebird (Menura novaehollandiae), olive-backed oriole (Oriolus sagittatus), Australian magpie (Gymnorhina tibicen), currawongs (Strepera spp.), honeyeaters (Meliphagidae), crows and ravens (Corvus spp.) and white-winged choughs (Corcorax melanorhamphos).Pavey, C. R., Smyth, A. K., & Mathieson, M. T. (1994). The breeding season diet of the powerful owl Ninox strenua at Brisbane, Queensland. Emu, 94(4), 278–284.Goth, A., & Maloney, M. (2012). "Powerful owl preying on an Australian brush-turkey in Sydney." Australian Field Ornithology, 29(2): 102. In one study, the most frequently killed bird prey species were pied currawong (Strepera graculina) and crimson rosella (Platycercus elegans) and avian prey taken as a whole was estimated to average . Rarely, other birds of prey have been killed and eaten, including large accipiters such as brown goshawks (Accipiter fasciatus) and grey goshawks (Accipiter novaehollandiae).Mourik, P & Richards, A.O (2019). Predation of a grey goshawk with a powerful owl as the likely predator." Australian Field Ornithology, 36: 5–10. Insects (such as crickets and beetles) and crayfish may supplement the diet and are typically taken on the wing. Rarely, lizards and snakes are also captured. It is also known to scavenge on carrion from time to time.