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Powerful Owl

Powerful Owl in Wombat Forest. Image Gayle Osborne

 

Powerful Owl
Ninox strenua
Kingdom:   Animalia
Phylum:   Chordata
Class:   Aves
Order:   Strigiformes
Family:   Strigidae
Status
Australia:  vulnerable 
Victoria:  vulnerable
FFG:  Action statement No. 92

 

Overview

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).

Distribution

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 2015.

 

Habitat and ecology

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).

Feeding

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

 

Home range

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).

Breeding

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

Threats

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.

Land use changes from residential or industrial development around the eastern fringe of Melbourne needs to protect Powerful Owl habitat.

Intensification of agricultural activities resulting in the clearance of suitable habitat can be detrimental to Powerful Owls.

 

Threat abatement measures across Victoria

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. 

Important Local Government Areas in Victoria for Powerful Owl

  • Ararat Rural City
  • Baw Baw Shire
  • Benalla Rural City
  • Cardinia Shire
  • Central Goldfields Shire
  • Colac Otway Shire
  • East Gippsland Shire
  • Glenelg Shire
  • Knox City
  • Latrobe City
  • Mansfield Shire
  • Maroondah City
  • Mitchell Shire
  • Moira Shire
  • Moorabool Shire
  • Mornington Peninsula Shire
  • Mt Buller Alpine Resort
  • Mt Stirling Alpine Resort
  • Murrindindi Shire
  • Nillumbik Shire
  • Northern Grampians Shire
  • Pyrenees Shire
  • South Gippsland Shire
  • Surf Coast Shire
  • Wellington Shire
  • Whittlesea Shire
  • Yarra Ranges Shire

 

Conservation actions in Victoria

Research

  • DELWP (Arthur Rylah Institute) will undertake studies on Powerful Owl diet in 2015.
  • Undertake telemetry studies to determine dispersal and recruitment of young birds into the established population, and movements and home range size of breeding adults.
  • Conduct surveys to determine the abundance and extent of Powerful Owls.
  • Monitoring protocols for Powerful Owl and other owls were completed in 2013.
  • Encourage Universities, Birdlife Australia and research institutes (e.g. Arthur Rylah Institute) to conduct research into the density of owl populations, impacts of current forest and fire management practices on nest site availability, prey density, recruitment, home range requirements and dispersal capabilities in all habitat types occupied in Victoria..

Management

  • Identify at least 500 Powerful Owl Management Areas (POMA's) on public land across the known Victorian range.
  • Develop or revise management prescriptions and zoning for areas of State Forest.
  • Delineate POMAs in the Forest Management Planning process. Monitor at least 50% of POMAs in Benalla-Mansfield, Bendigo, Central, Goulburn Broken, Central Gippsland, Dandenong, East Gippsland, Mid-Murray, Midlands, North East, Horsham, Portland and Tambo Forest Management Areas. Also monitor POMA's in the Mt Stirling Alpine Resort, Otway Forest Park, Port Phillip & Western Port CMA and Western Gippsland Plains Group.
  • Monitoring is targeted at  50% of POMAs in State Forest and 50% of POMAs in Parks or Reserves regularly to determine persistence of owls and breeding success.
  • Encourage and assist Municipal Councils to develop conservation mapping and GIS overlay systems within planning schemes to improve information on owl habitat and breeding sites across private land.
  • Encourage private landowners to enter into voluntary agreements (e.g. Trust for Nature covenants, Land for Wildlife Scheme) to protect owl sites on private land.
  • Provide input into regional fire management and operations plans through development of a comprehensive Natural Values Asset mapping tool for use by land managers dealing with fire.
  •  

Additional conservation measures in specific areas of Victoria

North East Victoria

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.

South West

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.

Port Phillip & Westernport

Mornington Peninsula - a survey was conducted in 2013. Six females and four males recorded for the area.

 

References

  • 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
  • Cooke, R., Wallis, R., Webester, A., (2002) Urbanisation and the ecology of powerful owls (Ninox strenua) in outer Melbourne, Victoria, In Ecology and Conservation of Owls Eds. Newton I., Kavanagh R., Olsen J., & Taylor I., CSIRO Publishing, Australia.
  • DSE (2004) Flora and Fauna Guarantee Action Statement No 92, Powerful Owl, Department of Sustainability & Environment, Victoria. link to DELWP FFG Action Statement No 92 pdf 230KB
  • Kavanagh, R. (2002) Comparative diets of the Powerful Owl (Ninox strenua), Sooty Owl (Tyto tenebricosa) and Masked Owl (Tyto novaehollandiae) in southeastern Australia, In Ecology and Conservation of Owls Eds. Newton I., Kavanagh R., Olsen J., & Taylor I., CSIRO Publishing, Australia.
  • Lavazanian, E., Wallis, R., Webster, A. (1994) Diet of powerful owls (Nixox strenua) living near Melbourne, Victoria, Wildlife Research 21(6) 643 – 645, Full text dx.doi.org/10.1071/WR9940643
  • Loyn,R., McNabb, E., Volodina, l., Willig, R. (2002) Modelling distributions of large forest owls as a conservation tool in forest management: a case study from Victoria, southeastern Australia, In Ecology and Conservation of Owls Eds. Newton I., Kavanagh R., Olsen J., & Taylor I., CSIRO Publishing, Australia.
  • McNabb, E.G. (1996) Observations on the biology of the Powerful Owl Ninox strenua in Southern Victoria, Australian Bird Watcher, 1996 Vol.16 (7), 267-295.
  • Seebeck J.H. (1976) The diet of the powerful owl Ninox strenua in Western Victoria, Emu 76(4) 167 – 170, Full text dx.doi.org/10.1071/MU9760167
  • Soderquist, T.R., Lowe, K.W., Loyn, R.H., Price, R. (2002) Habitat quality in Powerful Owl (Ninox strenua) territories in the Box-Ironbark forests of Victoria, In Ecology and Conservation of Owls Eds. Newton I., Kavanagh R., Olsen J., & Taylor I., CSIRO Publishing,
  • Tilley, S. (1982) The Diet of the Powerful Owl, Ninox Strenua, in Victoria, Australian Wildlife Research 9(1) 157 – 175, Full text dx.doi.org/10.1071/WR9820157

 

Further information

 

 

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