VOL. 16 (7) SEPTEMBER 1996 267 AUSTRALIAN BIRD WATCHER 1996, 16, 267-295 Observations on the Biology of the Powerful Owl Ninox strenua in Southern Victoria by E.G. McNABB, P.O. \IJ~~ 408, Emerald, Victoria 3782 . \. Summary A pair of Powerful Owls Ninox strenua was studied at each of two sites near Melbourne, Victoria, for three years (1977-1979) and 15 years (1980-1994 inclusive) respectively, by diurnal and nocturnal observation. Home ranges were.mapped, nest sites characterised and breeding chronology and success monitored. General observations at these and eight other sites, of roosting, courting, nesting, parental and juvenile behaviour, fledgling mortality, hunting, interspecific conflicts, bathing, and camouflage posing, are presented. The regularly used parts of the home ranges of two pairs were each estimated as c. 300 ha, although for one pair this applied only to the breeding season. One pair used seven nest trees in 15 years, commonly two or three times each (range 1-4 times) over consecutive years before changing trees. Nest-switching may have been encouraged by human inspection of hollows. Nest entrances were 8-40 m (mean 22 m) above ground. The owls clearly preferred the larger and older trees (estimated 350-500+ years old), beside permanent creeks rather than seasonal streams, and in gullies or on sheltered aspects rather than ridges. Laying dates were spread over a month from late May, with a peak in mid June. The breeding cycle occupied three months from laying to fledging, of which the nestling period lasted 8-9 weeks. Breeding success was 1.4 young per pair per year and 94% nest success; early nests in gullies were more successful than late nests on slopes. The post-fledging dependence period lasted 6-7 months from fledging. The diet consisted mostly of possums and gliders, primarily the Common Ringtail Possum Pseudocheirus peregrinus (78-89% by number). A resident breeding pair of owls took at least 95 major prey items in 368 days. Introduction The forest-dwelling Powerful Owl Ninox strenua is listed by the Department of Natural Resources & Environment (Victoria) as 'Rare', i.e. not at present endangered or vulnerable, but at risk. In order to avoid degrading of its current conservation status, appropriate management of its habitat is essential (Baker-Gabb 1993) and that management depends on sound knowledge of the owl's ecological requirements. Previous Victorian studies have addressed diet, behaviour and general biology of the Powerful Owl, e.g. Fleay (1944, 1968), Seebeck (1976), VanDyck & Gibbons (1980), Tilley (1982), Gibbons (1989), Hollands (1991), Quinn (1993 and in prep.), Traill (1993), Lavazanian et al. (1994), and Wilson et al. (1994). However, there has been confusion about the length of the nestling period, variously stated or implied in Schodde & Mason (1980) as about 5, 7 and 8-9 weeks. Other possibly contentious issues include aspects of the male's behaviour during the breeding cycle (roosting proximity/entering of the nest hollow), laying chronology, re-use of hollows, nest defence, home-range size, and importance of the Greater Glider Petauroides volans in the diet (cf. Schodde & Mason 1980, Chafer 1992). There have been little quantification of breeding parameters and few -observations of territorial defence, the post-fledging period or capture of prey. This paper describes, for several pairs in southern Victoria over 17 years, nesting apd fledging observations and data, nest-tree parameters, sex roles, length of post­ fledging dependence period, nest and territory defence, interspecific territories, diet, camouflage posing, roosting behaviour, hunting, and bathing. Study areas Olinda State Forest (3J05l'S, 145°23'£) Olinda State Forest, about 850 ha, lies between the eastern margin of the Dandenong Ranges and the Silvan Reservoir. Its northern edge abuts forested public land along Olinda Creek, the eastern boundary abuts forested water catchment and the western boundaries are dotted with semi-urban AUSTRALIAN 268 McNABB BIRD WATCHER settlement. The southern edges abut a large forest arboretum, public golf course and the National Rhododendron Gardens. Elevation ranges from about 250 m in the north to 565 m in the south-west. Average annual rainfall is from 1250 mm in the lower northern area to 1524 mm in the higher southern area. Some sections were selectively logged until 1968 and lightly utilised since (for electricity poles) until the mid 1970s. The area has since been managed for recreational use. This forest was severely burnt by fires in 1936, 1939 and 1962 but many mature specimens of Manna Gum Eucalyptus vimiTUllis and Mountain Grey Gum E. cypellocarpa have survived and are relatively abundant along the banks of Lyrebird Creek and its tributaries. Mountain Ash E. regTUlns is represented by small stands at the head of several gullies. The forested slopes comprise a mixture of Messmate E. obliqua, Narrow-leaved Peppermint E. radiata and Mountain Grey Gum, including many surviving pre-logging/pre-wildfire mature trees. Red Stringybark E. macrorhyncha and Broad-leaved Peppermint E. dives occur along the north-eastern boundary. Rough Treefern Cyathea australis, Soft Treefern Dicksonia antarctica, Silver Wattle Acacia dealbata, Prickly Moses A. venicillata and Black\\Qod A. meb:maxylon are abundant throughout. Autumn fuel-reduction burns are regularly conducted in some areas and slashing of ground vegetation occurs along roadsides. Femtree Gully (37°54'S, 145°]9'E) The. Ferntree Gully study area is within the former (until 1987) Ferntree Gully National Park. It is Victoria's oldest national park (113 years), now described as the Ferntree Gully Sector of the Dandenong Ranges National Park. The sector lies on the edge of some of Melbourne's long-established eastern suburbs where its western boundary abuts residential properties. The forested slopes of One Tree Hill form a buffer between them and the study area. On the southern edge a major road, Burwood Highway, separates the park from well-treed urban settlement which continues around the eastern boundary. The national park extends to the north where Mt Dandenong (633 m) stands 7 km away. Total area of the Sector is about 466 ha. Ferntree Gully Creek rises at the head of this glllly, at c. 475 m and flows southward to exit the Sector at c. 150m above sea level. Average annual rainfull during the study (1980-1994) was 1260 mm (W. Burnett, resident park ranger, pers. comm.). The creek banks and lower slopes are dominated by ancient Manna Gums and Mountain Grey Gums with an understorey of Blackwoods (some specimens over 20m high), Blanket-leaf Bedfordia arborescens, Musk Daisy Bush Olearia argophylla, Hazel Pomaderris Pomaderris aspero, ~ Cassinia aculeata and magnificent examples of both Soft and Rough Treeferns. A mature mixed forest of Mountain Grey Gum, Messmate and Narrow-leaved Peppermint covers most of the western slope, and a similar-aged forest of predominantly Mountain Grey Gum occurs in the north-eastern ('Tremont') tributary gully. Most of the eastern and south-eastern slopes were razed by wildfire in 1968. Some dense stands of regrowth Mountain Grey Gum surround the surviving older trees. Fuel-reduction burns are not conducted in the main gully and slashing is restricted to picnic areas and major roadsides. Methods A resident pair of owls at Olinda State Forest was studied from late 1977 to 1979 when the adult male disappeared, and the study was then moved to Ferntree Gully (Figure 1). The Olinda pair, with one fledgling, was first located on 30 September 1977. Regular visits (average 17 per month May­ October, 7 per month November-April) were made in order to locate roosts, to try to track the birds' movements and therefore map the territory, to locate the nest site and observe their behaviour throughout the breeding cycle. A resident pair of Powerful Owls in Ferntree Gully was regularly observed and monitored over 15 breeding seasons, 1980-1994. Observations at Ferntree Gully have concentrated on mapping of the territory, prey identification, locations and characteristics of nest sites, loyalty to nest sites, ascertaining the regularity and length of the nesting period, the fidelity and breeding success of the parent owls, the age at which chicks fledge, period of dependence of young and general hunting, roosting, courting, breeding and parenting behaviour. Three other pairs were observed less frequently elsewhere in the Dandenong Ranges National Park and another eightlpairs at Brisbane Ranges, Christmas Hills, Eildon State Park, Monbulk, Strathbogie Ranges, The Basin, Warrandyte State Park and Yellingbo, Victoria (Figure 1). The date of commencement of nesting, i.e. laying of the first egg, was determilied by the owls' behaviour. Regular day-to-day visits were made (when practicable) to observe the owls at their daytime roosts through late May and into June until nesting was confirmed. During this period the members of a pair usually roosted close together, side by side, or at least within 10-20 m of each other. This was consistent for all pairs observed. When the male was roosting alone it was suspected that the female had taken up residence in the nest hollow. The nest, if known, was then monitored at dusk for activity such as the emergence and/or re-entry of the female to confirm the start of nesting. If the nes~ tree was not known (from the previous year) the area was monitored at dusk, at which time VOL. 16 (7) SEPTEMBER 1996 Biology of Powerful Owl, Southern Victoria 269 T E c w y -""'lbourne BO R Fs L " Figure 1. Map showing Powerful Owl study area and nest locations in southern Victoria. B = The Basin, C = Christmas Hills, E = Eildon, F = Ferntree Gully, L = Lysterfield, M = Monbulk, 0 = Olinda, R = Brisbane Ranges, S = Sherbrooke, T = Strathbogie Ranges, W = Warrandyte, Y = Yellingbo. the male usually called his mate out or visited the nest.