Powerful Owl in Wombat Forest. Image Gayle Osborne
| Powerful Owl Ninox strenua | ||||||||||
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The Powerful Owl is symbolic as being one of the supreme nocturnal predators in the forests of south-eastern Australia, it is the largest species of owl in Australia and its presence as a high level predator may serve as an indicator of forest health because it is reliant on forest ecosystems that support abundant wildlife.
The Powerful Owl is closely related to the lesser known Barking Owl Ninox connivens and the Southern Boobook Ninox novaeseelandiae, all three being categorised as ‘hawk owls’, owls that do not have the typical facial disk of ‘barn owls’.
The Powerful Owl reaches a size of 67 cm for males and 58 cm for females which is substantially larger than the Southern Boobook Owl (30 cm) and larger than the Barking owl (40cm). Adult plumage is mostly brown on the head, back and upper wings with light coloured barring when viewed from the back, underparts are mostly white with horizontal brownish vee shaped barring (chevrons). Juveniles have more white plumage in front and on the head with obvious darker brown areas surrounding the eyes. The adult face is dark brown around the eyes with a lighter coloured forehead, the eyes are orange/yellow and the bill is black. The feet are yellow with black talons, the feet have a very strong grip for holding prey.
The Powerful Owl can be distinguished from the smaller Barking Owl which has vertical streaks rather than barring across its front (Juvenile Powerful Owls have some sparse vertical streaks but not as obvious as the Barking Owl). The Powerful Owl also has more barring across its back rather than the white spots of the Barking Owl.
Barking Owl (left), Powerful Owl (right).
The Powerful Owl occurs along the south-eastern fringe of mainland Australia from southern Queensland into New South Wales and Victoria. In Victoria it occurs across the eastern highlands extending into south-west Victoria. Most records occur in forested areas, in the south-west records exist in the following biorgeions; Central Victorian Uplands, Goldfields, Greater Grampians, Glenelg Plain, Otway Ranges, Otway Plain and Warrnambool Plain.
Historic distribution of all known records. Source: Victorian Biodiversity Atlas, Department of Environment, Land, Water & Planning, Accessed May 2017.
There is a strong relationship between the occurrence of Powerful Owls and their habitat. They are forest dwellers which rely on areas of old growth forests that contain mature, live hollow bearing eucalypt trees that can be hundreds of years old. In a study of Powerful Owls near Melbourne, McNabb (1996) found a connection between habitat quality, home range size, diet and breeding success with owls in less productive habitat occurring more sparsely and breeding less successfully on a more generalised diet. Optimum habitat elements for fostering high breeding productivity, include high densities of large, live (old growth 150+ years) hollow bearing trees in sheltered positions and orientations.
Modelling the distribution of Powerful Owls in Victorian forests by Loyn et al. (2002) found that Powerful Owls favour the drier forest types which have many live hollow bearing eucalypt trees in association with Blackwood Wattles, diverse habitats and extensive mature forest within 2 to 5 km.
Powerful Owls may share tracts of forest with Sooty Owls and Masked Owls, but habitat modelling data suggest that Powerful and Sooty Owls have different requirements despite a broad overlap in distribution. Powerful Owls favour the more open forest and broad gullies, with plants such as Blackwood Wattle whilst Sooty Owls favour the wetter sites and rainforest with plants such as Silver Wattle, Blanket-leaf and Tree-ferns (Loyn et al. 2002). There is virtually no overlap between diets of Powerful Owl and Masked Owl, as they tend to forage in different places and take different prey. The Powerful Owl hunts for prey that almost exclusively lives in trees and mostly takes prey which is 50-100% of its own body weight whereas the Masked Owl takes smaller prey which is 3-20% of its body weight (Kavanagh 2002).
The powerful owl is a generalist hunter, preying on the most available prey at a given site and in a given season (Cooke et al. 2006). The main component of the Powerful Owl diet across its range is Ringtail Possum, this may be supplemented by other arboreal possums and gliders depending on the geographic location and prey present, eg Greater Glider, Brushtail Possum, Sugar Glider, Yellow-bellied Glider (Cookeet et al. 2002). Birds, insects and vegetation comprise only a small part of the diet and ground dwelling mammals are rarely taken (Tilly 1982, Lavazanian 1994 , Kavanagh 2002, Cooke 2006). The rate of feeding probably depends upon the size of prey taken, McNabb (1996) estimated a pair would take a major prey item eg. possum every 3-4 days. Seebeck (1976) estimated a breeding pair might take as many as 250 possums per year.
Powerful Owls feeding on Ringtail Possum in Wombat State Forest. Image: Gayle Osborne
Summary of diet from sites in south-west Victoria (from Seebeck 1976, Tilly 1982)
| Location | Main prey | Secondary prey |
| Point Addis | Ringtail Possum | Australian Magpie, Pied Currawong |
| Beaufort | Ringtial Possum & Australian Magpie | Pied Currawong, Sugar Glider, Brush-tailed Phascogale |
| Naringal East | Ringtail Possum | Sugar Glider |
| Linton | Ringtail Possum & Sugar Glider | |
| Aireys Inlet | Ringtail Possum |
The extent of home range is influenced by habitat quality and subsequently the abundance of prey. In areas near Ferntree Gully, east of Melbourne where there was good habitat and an abundance of Ringtail Possum prey the home range was estimated to be 300ha (McNabb 1996), for areas in the Box-Ironbark forests of Victoria the home range could be from 1380 to 4770ha (Soderquist et al. 2002), but it is generally accepted to be about 1,250 ha or within a 2 kilometres radii (Loyne et al. 2002).
Nest sites are selected high up in old, large living eucalypts (150 + years old). Nest hollows are large and can be about 1metre deep with an entrance nearly .5 m wide (Cooke et al. 2002). Owls may sometimes re-use nest sites or select a different hollow every 2-4 years. In Victoria it has been observed that laying takes place late May to a peak mid June with a nestling period 8-9 weeks during which one or two chicks are reared, post fledgling dependence lasts for 6-7 months (McNabb 1996).
Powerful Owl chick (above) with adult (below). Image: Gayle Osborne
A major threat to the Powerful Owl is a loss of suitable large hollow bearing trees which has a direct impact on the availability of nest sites and also reduces habitat that supports arboreal marsupials which comprise the majority of the owl’s diet.
Processes such as fire, clearing and thinning may lead to the direct loss of habitat trees or changes to the surrounding forest structure which may compromise the value of nest sites. Continual changes to forest structure may also reduce the availability of habitat for prey species such as Possums. Continual forest disturbance may also compromise the growth of replacement old growth (150+ years) trees which may never reach full maturity.
Loss of canopy vegetation may exposes mature eucalypts and render nesting sites unsuitable. Wide spread and frequent fuel reduction burning may thin out thickets of vegetation that support Ringtail Possums and other prey.
Intensification of agricultural activities resulting in the clearance of suitable habitat can be detrimental to Powerful Owls.
Creation of new reserves in Victoria, e.g. Great Otway National Park and Otway Forest Park combined with the cessation of commercial logging many areas of State Forest, e.g. Wombat State Forest has increased security to Powerful Owl habitat.
Powerful Owl Management areas (POMAs) have been identified across a variety of Crown land tenure to protect Powerful Owl habitat. For State Forests where clear-fell harvesting is used, areas of suitable habitat of at least 500ha (dependent on habitat type) are protected as SPZ within a 3.5km radius. Where selective harvesting is used, POMAs comprise Special Management Zones of about 1 000 Ha. In existing conservation reserves POMAs contain at least 500ha of continuous suitable habitat.
Conservation of the Powerful Owl will involve identifying at least 500 Powerful Owl Management Areas (POMA's) on public land across the known Victorian range.
Areas of private land that support Powerful Owls require protection with landholder support and through identification in municipal planning schemes.
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Management
Goulburn Broken area - avoid the development of intensive recreational facilities near known nesting and roosting trees and discourage access to breeding areas.
Mid-Murray Area - conduct survey work in the extensive riverine Red Gum forests along the Murray River to determine if this forest type is good quality Powerful Owl habitat (in State Forest and Conservation Reserves).
Mt Buller and Mt Stirling Alpine Resorts - protect Powerful Owl nest and roost sites from mechanical disturbance during fire suppression activities.
North West Victoria
Bendigo Forest Management Area - undertake a field survey at priority locations (including existing POMAs and SPZs) to establish the distribution of individuals and breeding pairs as well as key habitat features (e.g. roosting and breeding sites) by June 2014.
In 2015, reduce the threat of habitat loss (especially the loss of large hollow-bearing trees) through the Large Owls of the Goldfields Project (field survey, determination of priority habitat zones, mapping and creation of BOMAs, SPZs and SMZs.
Dandenong Forest Management Area - conduct radio telemetry study to investigate dispersal of young Powerful Owl from natal territory and its survival.
In 2012 Deakin University undertooksurveys using owl playback and GPS tracking of captured owls tagged with transmitters.
Deakin University PhD studies into Owl genetics in Yarra Valley completed.
Avoid the development of intensive recreational facilities near known nesting and roosting trees and discourage access to breeding areas.
Parks Victoria undertakes monitoring of breeding sites on land managed by Parks Victoria e.g. Plenty Gorge (2 sites), Warrandyte State Park (4 sites), Yarra Valley Parklands (2 sites), One Tree Hill Reserve, Kinglake NP (3 sites), Dandenong Ranges NP (~3 sites), Lysterfield SP ( 2 sites).
East Gippsland
DELWP Forests and Parks Victoria conduct large forest owl surveys in conservation reserves of East Gippsland.
Parks Victoria ensures the development of intensive recreational facilities near known nesting and roosting trees is avoided along with discouraging access to breeding areas.
Protect Powerful Owl habitat from intense and frequent burning. Protect nest and roost trees (wherever possible) from fire.
Horsham - in 2015/16 DELWP plans to undertake owl surveys as part of the West Regional Forest Agreement to improve estimation of population size and the location of breeding population.
Midlands - due to the ceasing of commercial logging operations in the Wombat Forest impacts on habitat are addressed through Wood Utilisation Plan and Fire Operations Plan.
Mornington Peninsula - a survey was conducted in 2013. Six females and four males recorded for the area.
Further information