Observations on the Biology of the Powerful Owl Ninox ...

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 postfledging 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

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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 Stringy bark 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 MayOctober, 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

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Biology of Powerful Owl, Southern Victoria 269

T

E

c w

-""'lbourne y

BO " 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. The first egg was assumed to have been laid during this first 24 hours (as for Southern Boobooks N. rwvaeseelandiae and Barking Owls N. connivens; Fleay 1944). The hatching date was assumed to be 35 days later (from Schodde & Mason 1980). Behaviour of the parent owls sometimes provided confirmation, e.g. the female's reluctance to leave the nest and/or soft vocalisations uttered within the nest.

The date when the owlet (the first, if two) left the nest and roosted outside in the nest tree ('branching') or elsewhere, e.g. another tree or on the ground (from now referred to as 'fledge date'), was ascertained when practicable by regular observations during the day and/or night.

The limits of the home ranges were determined by: (a) visual observations (bird seen at night); (b) aural records (bird heard calling, distance/location estimated); (c) survey along geographic and topographic bOundaries, noting the presence of other Powerful Owls

in neighbouring territories, occasionally by using taped calls; (d) reports from other observers, e.g. management staff; (e) the abOve information was marked on a map and the relevant area shaded. A transparent sheet,

marked with 1 ha grids, was laid over the territory map and the 'shaded grids' totalled.

The adult owls were sexed by: (a) size difference, the male being the larger; (b) voice, the male's voice being generally of a lower pitch than the female's; (c) 'physique' (when fluffed up the male's breast feathers give the bird a large-chested appearance

whereas the female may appear slightly 'pear-sliaped' because her abdominal feathers tend to fluff up more than those of the male, possibly an adaptation for brooding);

(d) a combination of the above; (e) during nesting, identification of the female was sometimes aided by visible signs of brooding,

i.e. scruffy abdominal feathers and/or abraded tips of tail feathers.

Nest trees were measured for -circumference at breast height (1.3 m abOve ground) over bark, with a 10m tape-measure. This was converted to diameter at breast height over bark (DBHOB). Nestentrance height above ground was measured with a clinometer. Nest-entrance aspects and nest-tree site aspects were recorded as a compass bearing by means of a 'Silva' orienteering compass.

Preliminary observations A pair of Powerful Owls was first located, on 30 September lfJ77, roosting in a

Messmate in a side gully of Lyrebird Creek, Olinda State Forest. The male was perched 10 m above ground, clutching the rear half of a Common Ringtail Possum Pseudocheirus peregrinus. His mate was 2m below him, and one fledgling was roosting 7 m from them in an adjoining Messmate.


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This owl family was tracked/monitored during the following months, but after 1 December 1CJ77 the birds were not seen or heard in the Lyrebird Creek gully until March 1CJ78. When weather conditions were dry and calm, the adult Powerful Owls were heard from higher points overlooking the gully, giving their territorial woo-hoo calls. These calls came in five- to ten-minute sessions for up to about an hour. On 24 March 1CJ78 the moon was full and an owl called continuously. On the following night, in similar weather conditions, the owls were quiet until a few minutes after moonrise, when they. called continuously for 40 minutes.

On 13 April 1CJ78 both male and female were calling just before dawn, and a search of the area at first light located them roosting side by side in the same Messmate in which they were first seen on 30 September 1CJ77. Their roost sites were not located again until 21 May 1CJ78, when they roosted side by side on a horizontal branch of a Messmate 75 m north-east of the former roost. The male held the hindquarters of a Common Ringtail Possum (Plate 50). The area was visited 12 times over the next month to try to ascertain when and where the female occupied the nest.

On 7 June 1CJ78 the male was roosting alone for the first time, and met the female in the nest tree at dusk. The nest hollow was in a branch of a giant creekside Mountain Grey Gum, 40 m above ground level. It was assumed that nesting started on the previous evening, 6 June.

Most other observations, and the bulk of this paper, refer to the pair living at Femtree Gully.

Courtship and mating Courting was observed to begin during April in Victoria and an early sign of this

was the male and female roosting closer together, in neighbouring trees or the same tree. As nesting became imminent they often roosted side by side. The male sometimes perched all day on the prey remains, e.g. a headless Ringtail Possum. The roosts used at this time were usually within 50 m of the nest tree. The courtship and mating phase occupied about two months.

Copulation was observed on at least eight occasions during the study. If the owls had roosted in separate trees, mating was sometimes preceded by duetting, beginning with soft single hoots, uttered call for call. These developed into a short duet of woohoo for woo-hoo calls lasting for up to about a minute before they then met in the same tree. Copulation was timed on three occasions and took 25-30 seconds.

An apparently 'half-hearted' attempt at copulation was observed at Olinda during April, two months before nesting. On 8 April 1979 the owls roosted in neighbouring trees c. 100m from the nest tree. They were at the same level, 3m apart. They became active at 1805 h by stretching wings and spreading their tails. At 1821 h a single herhoo call was heard before both birds shook themselves vigorously to shed rain-water from their feathers. A~ they met in a young Messmate, a soft hooting duet developed and they took turns at preening each other's faces. The male then hopped high onto his mate's back and balanced there in an apparent attempt to copulate. The female made a series of loud rabbit-like squeals. This act took 10-15 seconds and the male hopped back on to the limb beside her and, from there, flew off rapidly through the forest with his mate pursuing him, almost touching his tail in a 'slip-streaming' fashion. Copulation could not in this case have been successful because the male had been too high on the female's back.

True copulation occurred as early as 23 days before egg-laying (e.g. Olinda, 12 May 1979) and became a regular nightly activity as nesting became imminent. The Femtree Gully owls mated twice within 30 minutes on 28 May 1980, a week before laying,

VOL. 16 (7) SEPTEMBER 1996 Biology of Powerful Owl, Southern Victoria 271

Pair of Powerful Owls roosting in Messmate at Olinda State Forest, 21 May 1978. Male is clutching remains of Common Ringtail Possum.

Plate 50 Photo: E.G. McNabb

having roosted 7 m apart in a Blackwood. At dusk (1655 h) the female awakened slowly by stretching and preening. She then moved to another limb and called in soft single-note whoa calls. He stretched and called whoa softly in reply before flying across to her side. After they had nibbled gently at each other's heads and faces, she adopted a submissive pose and he carefully climbed onto her back. Mating was accompanied by a soft simultaneous duet of murmuring whoa calls and again faded amid a crescendo of rabbit-like squeals. These squeals seemed to indicate excitement and were uttered at every mating observed. (Similar squeals were also often heard when food was passed from one owl to the other.) The male then dropped from her back and flew a short distance away. Twenty-five minutes later he returned and immediately mounted her again. On this second occasion, I heard no sound from him but the female uttered the squealing again toward the completion of copulation. Both owls perched quietly for about 30 minutes, nibbling at each other's faces before the male flew off down the gully. Two nights later, on 30 May, they were observed mating twice w~thin 29 minutes.

On 21 May 1981, at Femtree Gully, the male was roosting in his usual Blackwood. The female was on another branch in the same tree with the rear half of a Ringtail Possum. As darkness fell he moved to her side and showed some interest in the food , but she seemed unwilling to part with it. After 3 minutes he flew to a nearby Blackwood mid started to call woo-hoo in a deep voice. The female carried the food in her talons as she flew to his side, then resumed eating. The male sat quietly beside her for 20 minutes as she continued to eat. Then, while she was still eating, he mounted her. The soft duetting whoo-whoo-whoo ... calls signalled the start of copulation and the rabbit-like squeals were heard as they finished. The male then immediately swooped down and away and was heard calling far off down the gully 10 minutes later. The female was still in the Blackwood, holding the food.

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Nesting behaviour

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During nesting the male usually roosted within 50 m of the nest tree, often holding the remains of the previous night's kill. Typically, before dusk he started to feed but sometimes stopped to call in gentle woo-hoo calls or single soft hoots a few times, then resumed eating. After eating up to half of a Ringtail Possum, or the equivalent, over a period of 15-20 minutes, in fading daylight he started to call more insistently to entice his mate from her hollow. Typically, she left the nest within minutes and flew to a fairly open 15ranch where he joined her, with the remaining posterior portion of the prey. They greeted each other with gravelly bleating and/or soft hooting. If the male did not offer the food willingly she would, although looking submissive, bleat persistently as she sidled up beside him waiting for her portion to be passed, from his beak to hers. Excited squealing was heard as she took possession, and the male then moved a short distance away to clean his beak and scrabble his talons on a branch and preen.

The female appeared less relaxed about her meal and usually finished eating within 15-20 minutes, cleaned her talons and beak, and returned to the nest. She rarely stayed out of the nest for more than 25 minutes during the early stages of incubation. As hatching became imminent she was rarely absent from the eggs for more than a few minutes. At this time she was not easily enticed out by her mate unless she was hungry, in which case she gulped a few mouthfuls and returned to the nest with urgency. There were also occasions when she took food into the nest and apparently ate inside.

When the female did not respond to her mate's food-call, he was observed sitting on a branch opposite the nest entrance, calling persistently for up to 10 minutes. This calling started with the normal deep-pitched, soft woo-hoo and gradually became higher and louder when she did not respond. These calls finally ascended to a higher, femalelike pitch and his voice sounded to be 'breaking'.

There were also times when the male had no food but still called his mate out at dusk. They greeted each other with soft bleating or hooting, then engaged in some mutual preening after which the female resumed incubation in the nest. The female's reluctance to leave the nest faded as the chicks grew larger and more able to maintain their own body heat.

Male entering nest

Males were observed to enter the nest, and sometimes remain therein for some minutes, during the incubation and/or nestling periods on several occasions during this study, at both sites (i.e. different pairs). These visits were not always related to food delivery.

Three incidents at Olinda in mid to late August 1978 involved the first evening activity at the nest, at dusk. In the first case an adult (sex unknown) entered the nest then, 4 minutes later, the second adult followed the first into the nest. Both were in the nest for 2 minutes before one, probably the male, emerged and perched in the nest tree before flying away. In the second case the female was calling woo-hoo in a nearby tree when the male alighted on th~ nest opening and poked his head inside, looking around, before entering the hollow. Several minutes later the female perched near the hollow, then a low grumbling noise was heard before she entered the nest. The male emerged immediately and perched outside on a lower branch before flying up the gully. In the third case the female had roosted outside with the male, then flew to the nest entrance to peer in; she moved to a nearby branch as the male called briefly before he too arrived at the nest entrance. He paused for a minute to look inside, and entered awkwardly, with wings high, dragging on the hole sides as he


lowered himself into the hollow. Two minutes later his head emerged for a thorough scan of the surrounds before he glided away. Faint trilling (of young) could be heard from the nest.

Three incidents at Ferntree Gully also involved the first evening activity, at dusk. In the first case (5 October 1987), the male met the female (which had roosted outside) on a dead branch beside the hole and then both moved to the entrance. An owlet could be heard trilling excitedly as one adult (sex unknown) entered the nest; the second adult appeared keen to follow but only managed to fit in its head and shoulders. In the second case (15 June 1988), the food-calling male approached in stages then finally flew to the nest, carrying food, and disappeared inside. He emerged 5 minutes later, without the food. The female was not seen or heard but was probably in the nest. In the third case (15 September 1994) the female was at the nest, with her head inside. She was dislodged and glided away when the male pushed his head and shoulders up from within the nest. He sat looking around for a few minutes, with his top half protruding from the entrance, then flew toward his mate. An owlet, the first of two, was 'branching' well out on a limb at the same level as the nest.

Nest and territory defence At Olinda on 16 May 1979, at 1810 h the female began calling from a tall Manna

Gum, high over the track. She was answered from the west by the deeper woo-hoo of the male which soon joined her on an exposed limb, carrying a Sugar Glider Petaurus breviceps. Several other Sugar Gliders were yapping vigorously in alarm. As soon as the male had consumed the head he passed pieces to the female, begging submissively by his side. At 1831 h they were still feeding when the distant woo-hoo calls of another female were heard from the south-east. The female interrupted her meal, wiped her beak on the branch a few times then flew off in the direction of the calls. She was heard to call six times from farther away, and fmally at the same distance as the 'interloper' . She had 'homed' in on this second female, which obviously was unwelcome in her territory. While she was gone the male waited on the 'feeding' branch holding the remnant piece of glider under his foot. After 40 minutes he called loudly 5-6 times without response then tucked his head into his shoulder and slept.

At Ferntree Gully on 28 July 1993, the male had roosted in a Blackwood 20m east of the nest tree, holding the posterior half of a Ringtail Possum. At 1728 h he began to call softly. Soon after, the female emerged from the nest hollow, looked around, and flew off to the north-east. She returned 13 minutes later and re-entered the nest. At 1750 h the male called deeply for a minute; flew to the nest tree (with the possum) and started his persistent woo-hoo call which usually summoned his mate out of the hole. She did not respond. When the male had been calling for about 5 minutes, another female Powerful Owl arrived in a Blackwood 50 m west of the nest tree uttering a strange, excited 'hut hut' variation of woo-hoo; this bird may have been a sub-adult from the previous season. The male immediately landed on the branch 1.5 m from her and hooted aggressively, whereupon the 'strange' female took off and flew at great speed down the valley with the resident male owl in close pursuit. During the next 45 minutes there was no further sight or sound of any of the Powerful Owls.

On three different evenings at The Basin during August 1993, a parent Powerful Owl defended the nest and chicks against a human observer. On 19 August 1993, at 1803 h, two owlets were trilling in the nest as their parents met outside. The female noticed me watching from c. 30 m away and glided down silently, directly at my head. She made no move to steer away when I waved my arms and hat at her as she approached. Just one such pass was made and she remained silent, not uttering


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the warning hoot which I had come to expect from an agitated Powerful Owl in previous similar experiences.

On 28 August 1993, at dusk while the female was busy feeding an owlet at the nest, her mate was distracted by my presence and flew low over my head twice. He made no sound and may have been just investigating rather than aggressive, but he seemed threatening.

On 31 August 1993• between 1245 and 1400 h (daylight), one owlet was branching a few metres from the nest hole while the female was roosting in a nearby gum, clutching half of a Ringtail Possum. The second owlet was not seen, and the male was roosting in a Narrow-leaved Peppermint, 30 m from the nest tree. A flock of c. 25 Sulphur-crested Cockatoos Cacatua galerita was screeching and swooping near the owlet (in the nest tree) but a small party of Laughing Kookaburras Dacelo novaeguineae was harassing the male owl, flashing past him, often within centimetres. He became ruffled and restless during the onslaught and left his roost when a Little Raven Corvus mellori joined the cockatoos. He flew about from tree to tree, looking agitated, then focused his attention on me and escorted me from the area by swooping at me repeatedly as I walked up a narrow track. This owl showed more concern about human presence than he did about the mobbing cockatoos and aggressive kookaburras.

During this study, adult Powerful Owls have swooped at a climber when their nest tree was being climbed in order to band the young (pers. obs.) . In my experience, the owls often give warning by uttering a soft single hoot as they are about to swoop; although these swoops have come close and been quite unnerving, such activity by an obviously agitated owl has not (to date, during this study) involved contact between owl and human.

Parental behaviour Female Powerful Owls twice showed guarding or 'shepherding' behaviour toward

large nestlings. At dusk on 29 August 1978, at Olinda, a nestling aged about 7 weeks emerged from the nest and sat on the edge of the entrance, 40 m above ground, bobbing and gyrating its head for c. 10 minutes as it surveyed the surrounding forest, possibly for the first time. The adult female arrived on a branch facing the chick and, making soft bleating sounds, hopped onto the entrance and used her breast to gently push her youngster backward into the nest. She did not enter the nest, but perched beside it for 9 minutes before flying away.

Similar behaviour was observed on 5 September 1980, at Ferntree Gully. The female had been roosting outside the nest during the day. At dusk she was seen emerging with half of a Common Ringtail. (It is not known whether this food was in the nest during the daytime.) She sat on a branch near the nest and was joined by the male. An owlet emerged from the hole and trilled hungrily. The male made an attempt to claim the food but was refused by his mate which re-entered the nest, carrying the food, pushing the chick backward into the nest as she did so. The male stayed in the nest area for about 30 minutes, occasionally flying back and forth uttering soft bleating sounds as if still hoping for food.

During nesting the female spent most of her time in the nest incubating and subsequently brooding. She also was the parent to most often enter the nest with food, to feed the young. This close bond was seen to continue after the owlets had fledged.

In the immediate post-fledging weeks, as the owlets' flying skills developed, the owls began to roost farther away from the nest site. The whole family was often located roosting close together, e.g . in the same or in neighbouring trees. The female parent usually roosted closest to the young. The male also was often seen roosting beside


his mate or an owlet or both, but he did, however, often roost some distance away, sometimes as far as 20-50 m away. There were also many occasions when the female and young were located at roost but the male was not in the area, roosting in another part of the territory. In many of these cases he arrived at dusk with prey and gave it to the female which fed herself and the owlets. It was obvious throughout the study that the female tended to stay close to the young, guarding them, while the male ranged farther in the quest for prey. She continued to supply some of their food after they were in adult-like plumage.

During summer, in the post-fledging period when the adult owls roosted farther apart (though usually within calling distance), the strong pair-bond was still demonstrated. The male called at night in December and January, summoning his mate and offspring to share food which he had captured.

Sole parenting

In 1979 at Olinda the Powerful Owls were nesting when, in late August, the male disappeared (died?) leaving the female to raise an owlet 'single-handedly'. Nesting had been proceeding normally with the male hunting and roosting alone while his mate brooded two nestlings. He was last sighted on 26 August, roosting alone. On 27 August the female was roosting in the same tree but the male was absent. Over the following weeks she continued to roost alone outside, and hunted for herself and the one surviving owlet. The cause of the loss of the other owlet is not known, but on 15 September its plucked flight feathers were found in the area.

Between 1800 and 1915 h on 15 September, a male Powerful Owl was calling nearby as the owlet was receiving food from the female. Between 15 September and 21 October the female and owlet were observed on 22 days, 18 times at roosts and 10 times during their active (dusk to dawn) hours. The male was not observed or heard again. A lone female was the only Powerful Owl located in the territory during the 1980 breeding season.

The rearing of an owlet by an (apparently) unsupported female at Olinda suggested that the adult male died sometime after 15 September. He may have been still alive and ranging farther away in pursuit of food, but this seems unlikely in view of the observation time spent (34 daylight and 20.5 night hours) in the territory without seeing or hearing him again after 15 September.

Fledging and dependence

Nestling raptors in general are known to suffer significant mortalities during their first weeks because of their age differential, i.e. because the eggs are laid at intervals of up to four days, depending on the species (Fleay 1968, Cupper & Cupper 1981). The older chicks have a physical advantage in competing for food. During this study the Powerful Owls showed parental behaviour which demonstrated that no such inequality occurs after fledging, and that although the female plays the major role of feeding young, the male does assist at times.

Soon after sunset on 17 September 1981 , at Femtree Gully, two fledgling owls were perched in different trees, about 40 m apart. The elder, which had left the nest on 15 September, a day earlier than its sibling, was joined by its mother carrying the posterior portion of a Ringtail Possum. The younger, weaker-flying owlet waited, trilling hungrily from its distant perch as the mother tore small strips from the possum and fed them to the youngster at her side. It seemed for a minute that only one of


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the owlets was going to be fed unless the second one flew to join its mother also. The male parent soon arrived beside the female and sidled up to her, making submissive soft bleating sounds. She responded by turning to pass food to him which he then flew the 40 m to deliver to his second hungry offspring. Over the next 10 minutes the male flew back and forth ferrying food from the female to the younger owlet. The female was passing food to the owlet on her left, then to her mate on her right.

Vulnerability of young

At nest-leaving age young Powerful Owls start to 'branch', i.e. venture from the nest hollow out onto a close branch where they have space in which to strengthen their flight muscles by performing vigorous flapping exercises. This activity also strengthens the leg and foot muscles used for perching. While in the nest they sleep sitting/standing on the floor, not actually gripping a solid perch. At Lysterfield in 1994 a healthy nestling of estimated age 7 weeks was lying on its side, apparently asleep, in a nest hollow before removal for banding (S. Craig pers. comm.). During branching they are therefore vulnerable and liable to fall, particularly when affected by strong winds or when attacked or mobbed by other birds. When on or near the ground these owlets are potential prey of terrestrial predators such as the introduced Red Fox Vulpes vulpes, feral or domestic dogs Canisfamiliaris (e;g. McNabb 1994a) and cats Felis catus and, historically, their counterparts Dingo C. familiaris dingo and Spotted-tailed Quoll Dasyurus maculatus. Two cases, one involving both owlets of a brood, illustrated this vulnerability.

On 5 September 1983 at Femtree Gully (nest B, Figure 2), an owlet was found on the ground. It showed no sign of injury so was banded and placed 2 m above ground in a shrub. The adult female was roosting overhead in the nest tree. Four hours later, at 1800 h the owlet was on the ground again, beside the shrub. The owlet was again placed 2 m up in the shrub and observed from a concealed position. At dusk the male arrived with prey (Ringtail Possum), food-called then fed the female, but neither adult showed any interest in the owlet beneath them, which was not begging. The owlet was still in place at 1900 h, when a fox was seen 200m from the owls' nest. On the following day, two bloodied primary feathers of the owlet were found near the site and a strong odour of fox was evident. The adult female was again roosting on a branch of the nest tree.

At 1300 h on 13 September 1983, the second owlet was on a thin branchlet 3m below the nest entrance. The female was roosting on a nest-tree branch and her mate was in a nearby Manna Gum; all three were soaked by rain. At 1800 h both adults were perched low near the nest tree and the owlet was on the ground beneath them. It was in good health so was banded and placed in a shrub 2 m above ground as both adults swooped low overhead several times, hooting. Over the next four days ( 14-17 September) the owlet; which looked healthy, had moved slightly higher in the same and then a neighbouring shrub 3 m away, with the parents guarding and defending it. The Ranger in Charge, D. Newton, agreed to close the walking track past the owls until the owlet was at a safe height. On 17 September C. Compton checked on the owls after dark and found the owlet on the ground. He placed it up on a branch 2m above ground. At 0800 h on 18 September, the owlet could not be found although both adults were guarding and defending the site. fresh fox faeces were found, and the fox odour was noted. The owlet was not seen again.

At the same nest site (B) , at 1200 h on 2 September 1984, the female adult was roosting, and two owlets were branching, c. 20m up in the nest tree. The male was roosting in a Blackwood, 30 m away. Strong winds were blowing throughout the


E F

Lyre hi

D D•pot c o_u

~-----rr.~~~n~t~~~.~.~G~u~l~l~y<c~~~•=•~k---

----;;. ~o~th

Approx. soal•: L....._____J J.aa M

Figure 2. Map showing Powerful Owl nest sites at Ferntree Gully, 1980-1994, and distances the owls moved from site to site. A: 1980, 1981. Distances: metres B: 1982, 1983, 1984, 1985. A-B = 700 C: 1986, 1987, 1988. B-C = 150 D: 1989, 1990. C-D = 490 E: 1991. D-E = 200 F: 1992. E-F = 130 G: 1993, 1994. F-G = 300

day. At 1800 hone owlet fell ('parachuted') about 15m and arrested its fall by grasping a lower branch of the nest tree. Both adults defended by flying over and hooting. The next morning, both owlets had descended closer to the ground: one c. 10m up, on a nest-tree limb, the other atop a 2 m dead shrub at the base of the nest tree. This owlet was easily captured in a hand-net, examined, banded and placed 2.5 m high in a shrub beside the nest tree. The female hooted and descended (from her roost in the nest tree) to watch from a branch 7 m above me but did not swoop. By dusk the banded owlet had clinibed to the top of the shrub and was perched on a flimsy twig. At 1630 h the following day it was perched securely on the top of a broken shrub tr:unk, 4 m above ground. Its sibling was 15 m up in a nearby eucalypt. The female had been roosting in another neighbouring eucalypt, holding three-quarters of a Ringtail Possum. At 1720 h she flew to the side of the higher owlet and commenced to feed it. The banded (lower) owlet was trilling softly, but remained in situ. Next day (5 September) the banded owlet had jumped/flown about 2 m to a horizontal branch. The other owlet was with its mother, 18m overhead. Over the next two days the banded owlet progressed 6 m to the top of a Dogwood, then 7 m high in a Blackwood. The other family members kept roosting within view of it. On 8 September the female and banded owlet roosted together, c. 10m up, in another Blackwood, and the male with the other owlet 12m up in a Manna Gum 20.m away. Both owlets were last seen on 11 September. On 12 October a few plucked flight feathers of an owlet were found 200 m east of the nest tree; it bad apparently been eaten by a fox. Weather conditions had been severe from 16 to 19 September with heavy rain, which may have been a major factor in the owlet's death.

278 McNABB

Table 1

AUSTRALIAN BIRD WATCHER

Powerful Owl nest parameters, southern Victoria, 1977-1994. n/a = data not available, DBHOB = diameter at breast height over bark, NEH = nest entrance height.

Years Site Location Eucalyptus DBHOB NEH Site Nest used species (em) (m) aspect aspect

1977-79 1 Olinda cypellocarpa 233 40 creek w 1980-81 A Femt;ee Gully viminalis 154 22 creek E 1982-85 B Femtree Gully cypellocarpa 171 25 creek s 1986-88 c Femtree Gully cypellocarpa 157 25 E N 1989-90 D Femtree Gully cypellocarpa 183 22 Eb E 1989 2 Olinda cypellocarpa 179 40 creek w 1989 3 Eildon bicostata nla 13 SE N 1991 E Femtree Gully cypellocarpa 171 23 Eb sw 1991 4 The Basin cypellocarpa 154 24 creek w 1991 5 Christmas Hills obliqua 88 15 creek• w 1991 6 Warrandyte dead n/a 10 river w 1992 F Femtree Gully cypellocarpa 138 25 Sb s 1992 7 Christmas Hills obliqua 113 16 creek• NE 1993 Gl Femtree Gully viminalis 216 21 creek s 1993 8 The Basin viminalis 138 19 creek sw 1994 G2 Femtree Gully viminalis 216 30 creek N 1994 9 Lysterfield viminalis 145 14 creek• N 1994 10 Lysterfield dead 101 8 s s 1994 11 Sherbrooke regnans 193 20 s s 1994 12 Monbulk cypellocarpa 221 35 creek sw 1994 13 Yellingbo viminalis 122 20 creek s • = seasonal, often dry b = exposed upper slope

Nest-tree characteristics

Femtree Gully At the Femtree Gully territory seven nest trees were used during the study period,

15 breeding seasons (Figure 2, Table 1). Two were Manna Gums and five were Mountain Grey Gums. Trees A, B and G were creekside trees, C and Don the lower sheltered east-facing slope, within 100m of the creek. TreesE and F were high on the east-facing slope, c. 300 m from the creek. The owls nested a maximum of four times in any one tree (B, 1982-1985). Two nest trees were used only once; these were the Mountain Grey Gums at the higher hillside sites, E and F. Otherwise they nested twice in each of three trees (A, D and G, though in different hollows in G) and three times in one tree (C, 60 m from the creek). All multi-use nest trees were at creekside (A, B and G) or within 100m of the creek (C and D). Multi-use trees were used in consecutive years before the owls moved to a new tree.

The reason for the owls to move up to the higher, less sheltered ground (trees E and F) to nest in 1991-1992 is unknown, but a pair of Sooty Owls Tyto tenebricosa nested in the gully during late 1990 (pair with fledged young seen in January 1991). This species had not been recorded in the area since 1937 (pers. obs.; Wheeler 1972), and is still resident at the time of writing. The two species may compete for tree hollows, therefore the Powerful Owls may have been displaced by the Sooty Owls. However, as the former returned to nest in a creekside Manna Gum (G) in 1993 and 1994, it would seem that both species were sharing the gully (see 'Interaction with other owl species' , p. 285).


16

12

"' 10

"' ro u -~ 8 ;;; D

~ z

6

4

2

0 0

0-50


8

Diameter in em

• Nest trees D Nesting events

Figure 3. Powerful Owl in southern Victoria 1977-1994. Number of nest trees in each diameter class (diameter at breast height over bark, em) (n = 18) and number of nesting events per class (n = 29).

In 1993 a nest hole 21 m high in the side of the straight trunk of G was used, but in 1994 the owls nested in another hollow, at the main junction of the upper branches of G. The latter hollow was 30m high. The owls nested an average 2.1 times per nest tree and 1.9 times per nest hollow.

Nest-tree parameters

A total of 21 nest sites, including the eight at Femtree Gully (i.e. 8 nest hollows in 7 trees), was measured (Table 1). Almost all nest hollows were in living trees, within or below the foliage canopy and thus sheltered by it. The two exceptions were dead stags: tree 6 (Warrandyte) on a river flat and tree 10 (Lysterfield) on a gentle south-facing slope (Table 1); living trees with suitable hollows were scarce at both these sites, particularly around tree 6 where fire had burnt several hectares during the previous summer.

Diameter. The diameter of nest trees (n = 18) ranged from 88· to 233 em diameter at breast height over bark (DBHOB) (mean 160 em, median 156 em, standard deviation 40.6 em). The interquartile range (i.e. 50%) of the nest trees was between 134 and 186 em, with a further 25% in the 187-233 em class. An apparent relationship occurred between multiple utilisation and the larger diameter (older) trees (n=29 nesting events, Figure 3). Trees in the 51 to 150 em DBHOB classes were used an average of 1.0 times each but (the older) trees in the 151 to 250 em DBHOB classes averaged 2.0 nestings each.

Age. The ages of the trees in the above DBHOB classes were likely to have ranged from 150 to )500 years (from Ambrose 1982). For example, a Mountain Grey Gum of 102 em DBHOB was estimated by Ambrose te be 280-300 years old; extrapolation

280 McNABB

5-10 11 -15 16-20 21-25 26-30

Entrance height (metres)

• Nests in class D Nesting events

31-35


36-40

Figure 4. Powerful Owl in southern Victoria 1977-1994. Nest entrances (n = 21) classed by height (m) and number of nesting events (n = 32) per height class.

of Ambrose's growth curve suggests that trees in the above interquartile range are 350-500 years old, and trees in the larger diameter classes are )500 years old.

Height. Nest-entrance height (n=21 nest sites) ranged from 8 to 40 m (mean and median 22m, standard deviation 8.7 m). The interquartile range was from 15.5 to 25m. There was a relationship between multiple utilisation and entrance height (n=31 nesting events , Figure 4). Hollows in classes 5-20 m were used an average of 1.0 times each whereas hollows in the higher (therefore generally older trees) classes were used almost twice each, i.e. average 1.8 times per hollow.

Site aspect. For a total of 31 nesting events at 20 sites Powerful Owls nested beside or within 100m of permanent water 24 times , beside or within 100m of a seasonal watercourse four times, and on the side of a ridge, between 200 and 300 m from water, three times. Nests not in gully bottoms were on sheltered east- to south-facing slopes (Table 1) and thus protected from prevailing westerly winds.

Nest aspect. For 31 nesting events at 20 sites (one tree had two hollows used by the owls) , the entrance faced from south to west in 20 cases and north to east in 11 cases (Figure 5) . Entrances to fourteen of the hollows faced south to west, versus seven facing north to east. Nest entrances facing west or south-west were in trees in gully bottoms or on an east-facing slope (Table 1) . Site aspect and nest orientation combined to maximise protection of hollows from the weather.

Possible effect of nest inspection Nestlings were banded during their pre-fledging stage, i.e. age )45 days, when

practicable at Femtree Gully and other sites . Such interference should be considered

VOL. 16 (7) SEPTEMBER 1996 Biology of Powerful Owl, Southern Victoria

N

6

1

9

s

281

Figure 5. Powerful Owl in southern Victoria 1977-1994. Nest-entrance aspects for 31 nesting events.

when seeking a possible cause for the owls to move to a new site. Nest trees climbed for banding (see Table 1) were: A/1981. A new tree was used in 1982. Two food-exchange trees (dead acacias) near

tree A were felled in late 1981, which may have been a factor in the move to B for 1982.

C/1987. Same tree hollow was used in 1988. C/1988. A different tree was used in 1989. E/ 1991. A different tree was used in 1992. 4/1991. Same hollow was used in 1992. 511991. A different tree (7) was used in 1992. 6/1991. Not used again. F/1992. A different tree (Gl) was used in 1993. 7/1992. The owls moved back to 5 in 1993. Gl/1993. A higher hollow in the same tree was used in 1994. G211994. An attempt was made to climb but the nest hollow was not reached.

There were no desertions related to nest-tree climbing, and such were not likely because nest trees were climbed only when nestlings were well advanced, within c. 10 days of fledging. At several other sites where the nest tree was not climbed, the owls selected another tree the following year. Examples of these and the year in which they were last used are: 1 Olinda 1979, B Femtree Gully 1985, D Femtree Gully 1990, 2 Olinda 1989, 4 The Basin 1992, and 8 The Basin 1993. Although nesting at Femtree Gully in 1986 was unsuccessful, apparently because of dese~ion (cause unknown), the same site (C) was used succes~fully in the following two years.

282 McNABB

Table 2


\ Powerful Owl nesting chronology and success at Olinda and Ferntree Gully 1978-1994. Site: see Figure 2 for Ferntree Gully; start = day when nesting started (1-365), e.g. day 1 = 1 January; fledge = day when young fledged (1-365); duration = no. of days from 'start' to 'fledge' ; age =age in days at fledging, i.e. 'duration' minus 38 to give youngest possible age (from '35-38' , Schodde & Mason 1980); young = no. of young to fledge; * = estimate, based on observed behaviour (1987-1988) and/or countback (1985, 1989, 1991-1993) from knoWb fledge day based on mean duration for 1978-1979 (Olinda),

1980-1984 and 1994 (Ferntree Gully), i.e. 97; n/a = data not available.

Site Year Start Fledge Duration Age Young

1 (Olinda) 1978 156 251 95 57 1 1 (Olinda) 1979 155 255 100 62 1 A 1980 158 253 95 57 1 A 1981 158 256 98 60 2 B 1982 153 253 100 62 1 B 1983 151 254 103 65 2 B 1984 148 241 93 55 2 B 1985 161* 258 2 c 1986 n/a failed 0 c 1987 n/a 278* 1 c 1988 n/a 278* 2 D 1989 180* 277 1 D 1990 180 275 95 57 1 E 1991 176* 273 2 F 1992 176* 273 1 G/1 1993 158* 255 2 G/2 1994 162 255 93 55 2

Nevertheless, nest inspection remains a likely and perhaps important factor in switching of nest sites, and should not be discounted.

Nest loyalty at sites other than Ferntree Gully At Olinda State Forest a creekside Mountain Grey Gum (1, Table 1) was the nest

tree used in 1978 and 1979 but was not used again after the adult male disappeared. Because faeces and prey fragments were found beneath this tree in 1977 and the owls used similar post-nesting roosts in 1978-1979, it was assumed that tree 1 had also been used in 1977. In 1989 another creekside Mountain Grey Gum (2) 1.4 km up the same gully was used but was not used the following year (1990). At a forested gully near The Basin, a resident pair was observed nesting in a Mountain Grey Gum (4) in 1991 and 1992, but in 1993 they adopted a new nest tree, a Manna Gum (8) , 150 m away. The Basin nest trees were creekside, or within 50 m of the creek (see Table 1) .

Breeding biology

Laying dates During this study the owls nested annually with a spread oflaying dates over about

a month from late May to late June or (possibly) early July, with a peak in the second week of June (Table 2, Figure 6). The notable variations were 1984-1985, when nesting (estimated) commenced 13 days later in 1985 than in 1984, and 1992-1993, when it commenced an estimated (from known fledging dates) 18 days earlier in 1993 than


May 24-31

Biology of Powerful Owl, Southern Victoria

Jun 1-7 Jun B-15 Jun 16-23 Jun 24-30

Week

• Known D Estimated [=] Poss ible

283

Jul1-7

Figure 6. Powerful Owl nesting at Ferntree Gully 1980-1994, clutches (n = 12) initiated in weeks of May-July. Note: one clutch in each of June 8-15 and 24-30 are estimates and could have been laid in the previous week or following week respectively. ·

in 1992 (see Table 2). In some cases of 'late' fledging where the start date was estimated (not observed), the projected laying dates of late June/early July may have represented re-laying after initial failure (as suggested by the double peak of laying dates in Figure 6) .

Nesting duration and nestling period

Nesting dates were determined by observation (see Methods) . In 1978 the Olinda female was in the nest on or before 6 June. Two owlets were sighted but only one, apparently the elder, fledged on 9 September. In 1979 she started on 5 June and the survivor of two owlets roosted out ('branche~') on 13 September. Nesting periods for 1978 and 1979 were therefore 95 and 100 days respectively .

At Ferntree Gully accurate nest starting dates were recorded for years 1980-1984, 1990 and 1994. Accurate fledging dates were recorded for years 1980-1985, 1989 and 1990-1994. In 1986 the nest site was not located until 7 August. Nestlings were heard but for reasons unknown the owls abandoned this nest. This was the only season in which breeding was unsuccessful at Femtree Gully .

Using the known starting and fledging dates for years 1978-1979 at Olinda and 1980-1984, 1990 and 1994 at Femtree Gully , a median of 95 days was calculated for the total nest days , i.e. number of days from start of incubation until fledging of chick (range 93-103 days , mean 97 days ; n=9 nesting events , Table 2) . For 1987 the start day is unknown but nesting was confirmed on 15 June; the nestling was seen in the nest on 5 October, so fledged on or after 6 October. For 1988 also, the start day is unknown; the fledge date was on or after 1 October, when two owlets were still in the nest.


BIRD WATCHER

Nesting duration was ) 13 weeks, or at least 3 months, for nine nesting events. Therefore, allowing for an incubation period of 5 weeks (35-38 days, Schodde & Mason 1980), the nestling period was )8 weeks. Young fledged at a minimum age of 55 days (7 weeks, 6 days) after the projected or inferred hatching date (i.e. 93 + days less 38 days).

Breeding success

Overall breeding success for the two closely monitored pairs (Olinda and Femtree Gully) was 1.4 young per pair per year (24 young over 17 pair-years, from Table 2). Sixteen (94%) of 17 nesting attempts were successful; the cause of the only nest failure, which occurred during the chick stage, was not known. Broods (B) of two were as frequent as broods of one (B/1 X 8, B/2 X 8; mean brood size at fledging = 1.5).

At Ferntree Gully, the six latest (in the year) nesting events involved sites on relatively exposed high ground (C, D, E, F), on the hillside or ridge. The eight earlier (in the year) nesting starts were at the lower, more sheltered creekside sites (A, B, G) and were more productive, with a success rate of 1. 8 fledglings per year compared with 1.3 fledglings per year for the higher sites (Tables 1 and 2).

As noted in 'Vulnerability of young', some owlets died after fledging, therefore the number of young surviving to independence may be a truer measure 'of success. An important factor in fledgling survival was found to be the presence of understorey plants abutting the nest tree; these provide fallen pre-fledged and newly fledged owlets with sheltered low roosts and a means of climbing to avoid terrestrial predators (see also McNabb 1994a). Some fledglings which died may have been weaklings debilitated by air-sac mite, to which owlets sometimes succumb after fledging (Fleay 1968).

Post-fledging period

The young displayed long-term dependence on the parents. Observations showed that the parent owls supported their young until they were 8 months old and may even have tolerated their presence in the territory until as old as 9 months (i.e. 6-7 months post-fledging), possibly still supplying some of their food.

At 0515 h on 6 January 1981 at Ferntree Gully, a sub-adult, aged about 23 weeks, was observed trilling hungrily. At 0520 h the adult female began calling woo-hoo nearby and the juvenile flew to meet her. She was perched in a dead acacia, feeding on a Ringtail Possum. The owlet trilled incessantly at her side as she tore pieces from the possum and swallowed them. She relinquished a piece occasionally when the persistent juvenile reached over and took it from her beak. At 0540 h she flnished eating and the juvenile tried to take the remains, rump and tail; after 4-5 seconds the female relented and flew off up the gully, leaving the juvenile with the food in almost full daylight. It flew down into the forest understorey and was lost from view, but was located later in the day, roosting on a branch of a Hazel Pornaderris, clutching the Ringtail Possum remains.

On 15 February 1978, at Olinda, a sub-adult owl, aged about 7 months, was sighted in the gully. Its plumage was pale but the breast showed the typical brown chevrons of the mature bird. It was making the insistent begging trilling call of a fledgling, and was therefore apparently still dependent on its parents.

At Ferntree Gully an owlet estimated to have hatched on 7 July 1990 was observed in the company of its parents several times during March 1991, the last occasion being 19 March. Although it was at least 8 months old, in adult-like plumage, it was still trilling vigorously as do hungry, dependent owlets of the species. The last sighting was only a fortnight before the adults were expected to begin courting.


At 1810 hat Ferntree Gully on 20 April 1981 a young Powerful Owl, identified by its paler plumage, was located perched in a Manna Gum abutting the nest tree (A, used in 1980 and 1981, in which it had hatched 9 months earlier). It flew to a dead acacia, carrying the rear portion of a Ringtail Possum. Shortly after, the adult male arrived and perched near the young owl as the female adult began to call softly nearby. The parent owls flew to the nest tree where they engaged in some indiscernible activity at the nest hollow. This activity lasted for 1-2 minutes when they separated and the female flew off toward the west and the male sat in another dead acacia for 5 minutes before he too flew off, down the gully (south). The young owl had not changed position and was eating its possum at 1845 h, as the female was calling from the west. The male replied from down the gully. On the following morning, at 0610 h, this young owl was seen again in the nest-tree area, alone. At first light it flew off toward the north-west where it was later located at 1130 h, roosting in a Hazel Pomaderris c. 150m away, in a side gully.

Home ranges Mapping of the Powerful Owls' home range at Ferntree Gully was conducted over

the 15 years, 1980-1994. The owls were seen and/or heard in the gully in all seasons. The sheltered western (easterly aspect) slopes and lower southern section of the gully were favoured for roosting and nesting; the owls also used understorey shrubs as sheltered roosts during hot weather and/or strong winds. No major changes to the forest structure occurred at the Ferntree Gully site during the study. The owls were observed ranging over an area of 160 ha, and their total home range was estimated to be c. 300 ha.

Autumn fuel-reduction burning reduced the understorey density in some sections of the the Olinda owls' home range, which was also estimated to be c. 300 ha during the breeding season. However, as the birds could not be located during the nonbreeding months (December-February inclusive), their home range may have been larger, with parts used on a seasonal basis. Furthermore, without radio-tracking data the total area of both pairs may have been underestimated. Nevertheless, their home ranges were probably small because the extensive shrub layer and abundant old eucalypts provided excellent food and shelter for, and hence supported high densities of, the preferred prey species (see 'Diet'). The apparent difference in home-range size of the two pairs may be explained by differences in habitat quality and prey densities, the Ferntree Gully site being the richer (more old forest and hollows; no recent fires, hence more complex understorey: see 'Study areas' and 'Diet') .

Interaction with other owl species

Sooty Owl

The owls at Ferntree Gully shared their territory with Sooty Owls during the latter part of the study. The first time Sooty Owls were observed was on 20 January 1991. In the centre of the Powerful Owls' territory, a young Sooty Owl was heard making the harsh rasping begging call of a dependent owlet (indicating breeding in 1990). Broadcast of taped Sooty Owl calls attracted an adult Sooty Owl. A sub-adult Powerful Owl also arrived, apparently interested in the Sooty Owl trills, and watched from a treetop. At one stage the adult Sooty Owl was 4 m below the young Powerful Owl, looking down from the same tree. Since then both species have coexisted in the gully and a Sooty Owl territorial scream has often been heard, occasionally in the immediate vicinity of the Powerful Owls' nest. The Sooty Owls also seem to tolerate Powerful Owls in their nest area. Nevertheless, the following observations suggest some interspecific competition for nest sites and possible interspecific territoriality .


BIRD WATCHER

On 14 October 1994, in the centre of the Sooty Owls' area of activity, a loud, downscale scream was heard. On arriving to investigate, within two minutes, I foup.d an adult Powerful Owl perched on a low branch at what seemed to be the exact location of the screaming Sooty Owl. No Sooty Owl was visible. This incident may explain the erroneous belief that Powerful Owls were responsible for the terrifying screams described by our pioneer ornithologists (Fleay 1968). The Sooty Owl may have directed its scream at the Powerful Owl.

It was noted that when nests of both species have been located in a shared territory, the Powerful Owls nested on higher ground, either upstream or (as at Ferntree Gully in 1991-1992) well away from the creek, on the ridge. During 1989 at Olinda, Powerful Owls nested in a gully 950 m upstream from a Sooty Owl nest site. These Powerful Owls were audible from the Sooty Owls' nest and the Sooty Owls from the Powerful Owls' nest. At Ferntree Gully the Powerful Owls nested on a high ridge twice during the study, in 1991 and 1992. Although the Sooty Owls' nest was not located during this period, they were still active in the gully. In 1993 the Powerful Owls had returned to nest in a creekside Manna Gum, but the Sooty Owls' nest site was unknown. In winter and spring of 1994 the Sooty Owls were active at the lower end of the gully, c. 750 m downstream from the Powerful Owls' nest.

Another shared territory was found at Tynong North. At a disused sand quarry abutting Bunyip State Park, a female Powerful Owl was observed on 18 November 1993 and a pair of Sooty Owls at the same site on 14 January 1994. Powerful Owls and Masked Owls T. novaehollandiae had been observed at the site over many years (J. Weatherhead pers. comm.).

Southern Boobook

The Southern Boobook is a common breeding resident in the two main study areas, and appears to coexist with the Powerful Owls most of the year. Clashes have occurred when the Boobooks were defending nests or young, but have also occurred at other times. Three significant clashes were observed.

On 3 January 1981 at Ferntree Gully (see McNabb 1994b), the Powerful Owls were attracted by tape-recorded woo-hoo calls to ascertain whether their previous year's offspring were still resident in the parents' territory. Both parents and a subadult arrived in response. A Southern Boobook, which was subsequently found to be nesting nearby, attacked them vigorously, making a 'quack' call as it swooped at their heads. The adult Powerful Owls soon left but the sub-adult sat through the assault for 10 minutes, and was hit on the head once without incurring injury. The Boobook eventually returned to its nest tree.

On 3 January 1982 at Ferntree Gully, at 2130 h one of the two young Powerful Owls was perched in a dead tree above a walking track. A 'h, h, h, h,' sound was heard approaching from across the gully, and a swiftly flying Southern Boobook appeared and struck the young Powerful Owl on the back of the head, sending it tumbling from its perch. The Powerful Owl started to fly before hitting the ground and flew to a tree 100m up the gully. The Boobook disappeared immediately (K. McNabb pers. comm.).

On 2 June 1987 at Ferntree Gully, at 1735 h the female Powerful Owl began calling near the previous year's nest tree and was soon answered by the male hooting softly from the nest tree. She flew to join him (out of sight). After 5 minutes they flew off down the gully. Fifteen minutes later I located them at a picnic ground where they were perched high up on open branches of separate trees. One had just flown to perch with its mate when they were immediately swooped upon by a 'yelping' Boobook. One Powerful Owl took off and pursued the Boobook in two and a half


tight circles, of c. 3m diameter, climbing upwards toward the eucalypt canopy where they were lost from view. A minute later I observed the 'pursuer' Powerful Owl perched atop a tree near where it and the Boobook had disappeared. It appears that Southern Boobooks consider Powerful Owls as predators and these incidents of aggression probably represent defence of the Boobook's young.

Hunting

During nesting the male Powerful Owl was responsible for the supply of food . The female, when incubating and subsequently brooding, was reluctant to leave the nest and therefore had few opportunities to hunt. When the young had fledged and were free-flying, she still maintained a close watch over them whilst the male ranged farther away in search of prey. When successful he returned and signalled to the others, by hooting, that a meal was available.

When the female had resumed roosting outside, leaving the nestlings alone, she was able to contribute to the sustenance of the family. For example, on 8 September 1978 at Olinda the two owlets were due to fledge soon and one was perched on the lip of the nest hole when the male arrived beside it, carrying half a Ringtail Possum. He spent 8 minutes gently feeding small pieces of possum to the owlet, then dropped the rump and tail portion into the hole and alighted on a dead branch facing the nest. At this moment the female, which had been roosting away from the nest, arrived in a nearby Messmate, carrying a wriggling, squealing Ringtail Possum. She killed the possum with a crunching bite at the rear of its skull/neck area, and disappeared with it, straight into the nest. She emerged 25 minutes later, without food, and engaged in allopreening with her mate.

Capture of a Ringtail Possum

On 4 December 1990, at The Basin at 2130 h, an adult male Powerful Owl was seen sitting on a low trackside gum branch. When illuminated by the spotlight he retreated into the forest canopy slightly downhill from the track. At 2145 h there were muffled flapping sounds in the upper foliage of a Narrow-leaved Peppermint, accompanied by urgent squeals. By spotlight I saw the Powerful Owl dangling, back downward, clutching a Ringtail Possum in one foot. The total weight of owl and possum was supported by the owl's other foot which gripped a clump of small leaves and branchlets. The possum stopped squealing after 10-15 seconds and became still but its eyes still reflected the light from the spotlight. The owl dangled in this position, its back parallel to the ground, underparts facing the sky, clutching the possum in one foot and the leafy branchlets in the other for almost 3 minutes. It did not look to be a comfortable pose but the owl was obviously determined to retain its grip until the prey had completely expired. The owl's next move was to grip the possum's rump in its beak and fly to a nearby branch. On arrival at this new perch the owl again gripped the possum in its talons and commenced to bite its face and head. The owl then began to feed, starting with the frontal skull and over c. 20 minutes devoured the entire head. I was minimising any distraction I was causing by only using the spotlight for a few seconds every 3-4 minutes, and at this stage the owl disappeared in the darkness with the prey.

Hawking

An adult Powerful Owl was observed hawking for insects on 13 September 1993, at Ferntree Gully. At dusk, 1740 h, the adult owls met briefly at the nest tree. They were without food. The male flew off and was soon heard calling from down the


BIRD WATCHER

gully. One of the owlets took its first flight, 4 m to a dead branch in the nest tree where it remained, trilling and occasionally exercising its wings by flapping vigorously. The female, which had been sitting in the nest tree since the male departed, suddenly took off, accelerating with several rapid wing-beats then gliding quickly and silently toward the prey. Her legs were thrust forward at the last second, grasping the insect in one foot. She then alighted in a nearby tree, took the prey in her beak and flew to the side of the owlet, which accepted it with great excitement. As the light faded, the female appeared to be watching for another such prey item but did not repeat the hawking activity. Five nights later, on 18 September, she was observed flying out from a high perch, apparently attempting to capture an airborne insect, without success.

On a few occasions a Powerful Owl was observed pursuing a flying bird. On 18 June 1992, at Femtree Gully, the male was perched on a limb in the nest tree calling repeatedly woo-hoo when a Crimson Rosella Platycercus elegans flew past him in the fading light. The owl instantly took off in pursuit and the panic-stricken parrot disappeared from sight with him in a rapid tail-chase.

Sub-adult 'hunting '

At Femtree Gully on 5 January 1982, I was accompanied by R.H. Loyn. Two young owls, aged 6 months, were located at 2105 h, trilling from understorey phtnts beside a walking track. As we approached, they flew out and landed on a Dogwood branch 5 m from us, 2.5 m above the track, apparently to investigate the soft crunching sounds made by our feet as we crept along the gravel path. After some exaggerated head-winding and staring, one bird swooped down and appeared to take something off the path, its talons scraping the gravel. It disappeared into the shrubbery for 2 minutes, and then reappeared to join its sibling on the Dogwood branch. They both then stared intently at us again, winding and twisting their necks to extremes as they did. Next came a shallow swoop (of one bird) at head level, directly toward us. We bowed simultaneously as the owl glided just over us ~d landed on another low branch, 20m up the track, to tum around and resume staring at us. A few minutes later the second owl made a low dive toward the ground near our ankles, flew past our feet and continued up the path. At this point the owls gave up inspecting us and began to work their way slowly up the gully, trilling as they went.

At 2135 h on 24 December 1981 in Fern tree Gully, an adult (female) and two young owls were in trees over the track. One of the young (now in adult-like plumage) was perched on the same dead branch as a Greater Glider. They (owl and glider) sat 1.5 m apart, looking at each other for 2 minutes. The glider slowly moved out to the thin extremity of the branch and paused. After several minutes the owl followed it along the branch but stopped 3 m from it. Once again they eyed each other. The owl then flew to a higher position in a neighbouring tree, returned the glider's stare and watched it leap to another tree and disappear into the foliage. The owl's sibling and adult were out of sight during the encounter.

At 1820 h on 10 May 1981 at Fern tree Gully, on a ridge track bordering the main gully, Peter Trusler and I were watching a small dog (Miniature Fox Terrier) trotting away from us along the track. A large owl glided silently down from above and behind the dog, lowering its feet as it passed c. 50 em above the dog's back. The owl then pulled out of its glide and flew up to a trackside eucalypt. The dog immediately turned about and trotted back in the opposite direction, toward us, but ran into the bush when it noticed us. It seemed to have been unaware of the owl, which was identified seconds later as a first-year Powerful Owl.


Night vision


On 24 July 1980 at Femtree Gully, at 1545 h, the male owl dropped a headless Ringtail Possum from his roost 15 m up in a large Blackwood; he may have been disturbed by the process of pellet collection beneath him. At 1730 h, after dusk, he dropped down in stages, branch to branch, scanning the ground beneath him from each branch, until he was on the ground within a metre of the food (which had been left undisturbed) . Once on the ground he looked around for the possum for about a minute, then returned to the tree above. Although the possum was visible to me in the moonlight, the owl had failed to retrieve it.

In 1989 at Eildon, a pair of Powerful Owls nested and raised one young (McNabb 1994a). Although not noticed in the field, later examination of photographs revealed damage to the female's right eye which was apparently blind.

Together with reports of a one-eyed female (Johnson 1995) and a female with an 'abnormal' eye (Traill1993) surviving in their territories for two years, these incidents suggest that Powerful Owls use other factors besides vision , such as prey movement and sounds thereof, to locate prey. However, a visually impaired female may obtain much food from her mate.

Diet

Information for this study was collected by direct observation (day or night) of prey held at roosts or delivered to females/nests/young, rather than by pellet analysis which is yet to be completed.

Relative abundance of 11Uljor prey species

No formal spotlighting mammal survey was conducted at Olinda but incidental observations of major prey species for the period 1 May to 12 June 1979 were recorded. Ninety-three Ringtail Possums and 13 Sugar Gliders were observed, i.e. 88% and 12% respectively. Similarly, only incidental records were kept for Femtree Gully but for the period 17 September to 16 November 1981, 25 Ringtail Possums, four Greater Gliders and seven brushtail possums (Trichosurus spp.) were observed, i.e. 69%, 11% and 19% respectively. (Greater Gliders are rare at Olinda and Sugar Gliders are rare at Femtree Gully.)

Olinda

Of 40 major prey items seen at Olinda, ·the Common Ringtail Possum was predominant. The secondary prey species was the Sugar Glider, which was not recorded outside the autumn-winter months. Although this sample is small , a definite seasonal trend was noted. Three Sugar Gliders were taken during May-June 1979 and one in August 1979. One bird was recorded, a Crimson Rosella Platycercus elegans (Table 3).

Femtree Gully

At Femtree Gully the owls were observed 575 times over the 15-year duration of the study. They were seen in possession of.200 major prey items (Table 4) . The Common Ringtail Possum was again the most commonly taken species (89%), with juvenile brushtail possums taken occasionally, always in spring, when the owls were feeding young. Both Mountain Brushtail (Bobuck) Trichosurus caninus and Common Brushtail Possum T. vulpecula occur in the area but were not specifically identified

290 McNABB

Table 3


Major prey items of the Powerful Owl observed at Olinda, 4 October 1977 to 9 October 1979.

Species

Common Ringtail Possum Pseudocheirus peregrinus Sugar Glider Petaurus br eviceps Crimson Rosella Platycercus elegans Unidentified

Total individuals

Total

31 4 1 4

40

%

77.5 10.0 2.5

10.0

100.0

when held by the owls. As at Olinda, a single Crimson Rosella was recorded. The owls were estimated to have taken at least 95 major prey items in 368 days, or one major item (possum) every 3-4 days .

During the early years of the Femtree Gully study, low numbers of Greater Gliders occurred in the gully. It was usual to observe up to six of them during a 20-rninute walk through the heart of the owls' territory and at least another four occupied old Mountain Grey Gums along the western ridge boundary of the owls' home range. However, they declined in number and became uncommon during the latter stage of the study. The decrease may be attributed to the Powerful Owls (e.g. Kavanagh 1988, Chafer 1992, Traill 1993), but no records of Greater Glider occurred in the list of 200 major prey items. Predation by the Powerful Owls may have gone unrecorded, a likely possibility as glider numbers appeared fairly low to begin with and this species is slow to recover its numbers compared with the Ringtail Possum (e.g. Strahan 1983). Ringtail Possums were the more abundant prey species and were, not surprisingly, the major prey . The possibility that the resident Sooty Owls may have been preying on Greater Gliders has been considered, but analysis of Sooty Owl pellets has not confirmed this (McNabb & Quinn in prep.).

Prey characteristics

Observed prey items at both sites were species characteristic of the tree and shrub canopy, with no strictly terrestrial prey recorded, although some of the brushtail possums might have been taken on the ground (e .g. Strahan 1983). The juvenile brushtails were probably captured during sorties away from their mothers, as at such times they are clumsy and noisy (pers . obs.).

Although Ringtail Possums build dreys in shrubbery where the understorey is dense and hollows are scarce, they prefer to nest in tree hollows when available (e.g. Strahan 1983). Where the understorey is sparse, they are obligate hollow-nesters (e.g . Traill 1991). In both study areas here , dreys were also constructed amongst canopy foliage (pers. obs.). Although understorey shrubs are plentiful at Femtree Gully and Olinda, dreys are uncommon at the former but moderately common at the latter. (This may indicate that most, if not all, suitable hollows are occupied.) Ringtail abundance is related to the availability of nest sites (Strahan 1983), and the Ringtails demonstrated a preference for arboreal hollows (which are abundant) at both sites.

Vegetation in diet

At Femtree Gully on 14 November 1989 an owlet was seen eating small strips ofloose Manna Gum bark (McNabb 1994a). This behaviour may have been ingestion of roughage for pellet formation (e.g . Fleay 1968), but was more likely to have been


Table 4

291

Major prey items of the Powerful Owl observed at Ferntree Gully, 8 May 1980 to 22 September 1994.

Species

Common Ringtail Possum Pseudocheirus peregrinus Brushtail possum• Trichosurus spp. Crimson Rosella Platycercus elegans Unidentified

Total individuals

Total

177 11

1 11

200

%

88.5 5.5 0.5 5.5

100.0

•unspecified Trichosurus sp. i.e. T. vulpecula or T. caninus (both species were observed in study area)

play or some other 'distraction' for the owlet. Such behaviour may partly explain the presence of vegetation in pellets (e.g. Lavazanian et al. 1994, Wilson et al . 1994).

Male control of food It was observed that the male Powerful Owl usually maintained possession of and

had first choice of any food, throughout the year (not only when the female was nesting). For 68 daytime observations of the Femtree Gully owls roosting, between 1980 and 1994, the male was holding the prey on 56 (82 %), the female on nine, and a juvenile on three occasions. Some prey items held by the female or young may have been captured by the male and partly eaten by him, then given to the female for her share, then finally passed to any dependent young. For example at 0900 h on 16 September 1980, at Femtree Gully, both adult owls and the fledgling were all roosting in a Blackwood. The female was holding a Ringtail Possum. At 1530 h the male was holding the possum. At 1835 h he was feeding himself, occasionally passing a small morsel to the loudly trilling owlet at his side. When only the rump and tail section of the prey remained, the female moved in and pestered him until amid some rough gravelly bleating sounds she gained possession of it. She ate none but immediately offered this food to the owlet which took only 5 minutes to devour it. The tail was last to disappear, with several gulping swallows.

Other behaviour

Bathing On 6 July 1987 at Femtree Gully, at about 1830 h, a Powerful Owl was disturbed

bathing in a forest dam at the head of the gully. It then sat on a low, dead stump at the water's edge to dry and preen its feathers. On 16 October 1979 at Olinda, between 1830 and 1930 h on my arrival at a forest dam, strong wing-beats were heard rising from the water. By spotlight a female Powerful Owl was seen on a dead branch which protruded from the water. She sat shaking and quivering her feathers to shed the water, as is normal for most bird species after bathing. On 21 October 1979, at Olinda, I arrived at 1830 h to see the adult female feeding as the owlet trilled nearby. I moved to the dam c . 40 m away and waited. At 1850 h the female glided steeply down to some branches directly above me. She waited for 2-3 minutes and then flew down to partly immerse herself in the water for a moment before flying up onto a dead branch.

After discovering the owls bathing at the above locations, I spent several fruitless pre-dawn sessions hidden at the water's edge (at both sites). After many pre- and


BIRD WATCHER

post-dawn observations I have not seen any evidence of pre-dawn bathing (cf. Fleay 1944, 1968).

Camouflage posing At 0900 h on 13 September 1980, at Ferntree Gully, the adult male Powerful Owl

was roosting in one of his favourite Blackwoods, c. 15 m above ground. His mate and the fledgling were roosting in another Blackwood nearby. A lone Grey Currawong Strepera versicolor al'rived in the male's roost tree and commenced foraging 7 m from him. The owl immediately adopted a slim posture by sleeking his feathers close to his body, stretching his neck upwards and closing his eyes until they were mere slits. The owl maintained his pose and the currawong continued to forage for 2 minutes without showing any recognition of the predator. It then flew off and the owl relaxed back into his normal roosting posture. A lone currawong would not have been a threat to the owl, but could have alerted other currawongs if there were any nearby, thereby initiating mobbing or attack. The owl's posture was therefore an effective defence tactic.

On 6 January 1981, at Ferntree Gully a sub-adult had received food, Ringtail Possum, from its mother at 0540 h. It was located later, during daylight, roosting on a branch of a Hazel Pomaderris, clutching the Possum rump and tail. At one stage, as it was being photographed (Plate 49, front cover) it adopted a tall, slender pose, with head tilted skyward, and eyes almost closed. Having previously observed similar 'camouflage' posture by Southern Boobooks and Laughing Kookaburras when they have sensed danger, I looked above the trees to see a soaring Wedge-tailed Eagle Aquila audax. After a few minutes the eagle had moved out of sight and the owl relaxed back into its usual posture.

Discussion The results of this study generally confirm and extend previous knowledge of the

Powerful Owl's behaviour, biology and ecological requirements, and provide new data on breeding ecology. Most aspects of this study agree with information presented by previous authors for Victorian populations, e.g. on courtship and mating behaviour and timetable, sex roles and parental behaviour (cf. Fleay 1968, Schodde & Mason 1980, Hollands 1991). However, some aspects of this study differed from previous accounts.

The male has been said to roost away () 100 m) from the nest tree during the breeding cycle (Schodde & Mason 1980), but observations herein and those ofTraill (1993) show that the male roosts near the nest tree. The male is said not to enter the nest during the egg/nestling phase (Fleay 1968, Schodde & Mason 1980), but this study and Hollands' (1991) provide observations to the contrary. Victorian birds are said not to re-use nest hollows in consecutive years, supposedly unlike northern birds (Fleay 1968, Schodde & Mason 1980), but this study shows that re-use is common, at least in the short term. The rate of nest-switching in this study (and Fleay's?) may have been inflated by human inspection of hollows. Nevertheless, nest trees were also switched after eight nestings where no such inspection occurred. The laying peak has been given as the second half of May (Schodde & Mason 1980), but this applies to northern birds (e.g. Debus & Chafer 1994); the peak for southern birds is slightly later (as for the species generally: Olsen & Marples 1993). The supposedly precise inter-year laying punctuality in partiCular nesting territories was not upheld by this extended study (cf. Fleay 1944, 1968; Favaloro 1946). Nevertheless, ± 16 days is a narrow laying 'window' over 15 years. Bathing routines are more flexible than previously suggested (cf. Fleay 1968, Schodde & Mason 1980); bathing is probably performed as necessary, often in the early evening after a hot day.


Nest-tree and other breeding parameters generally agree with a previous, limited analysis for New South Wales (Debus & Chafer 1994). Breeding success [1.4 fledglings per attempt (see Table 2)] was greater than that for a small and possibly biased sample for New South Wales (0.8 fledglings per attempt, Debus & Chafer 1994), and for pairs in marginal habitat in central Victoria (c. 0.5 fledglings per attempt, Hollands 1991). The impression from this study is that pairs achieve high breeding success in optimal habitat, particularly in their preferred large, old trees in moist gullies.

This study confirms the nestling period as about two months ( cf. Schodde & Mason 1980), and supports previous impressions that the post-fledging dependence period is also long, lasting six or seven months post-fledging and sometimes up to the start of the next breeding season (see also Hollands 1991 , Traill 1993, Debus 1995). The consequence, including two months of courtship/nest preparation and a month for incubation, is a breeding cycle that occupies 10-11 months of the year.

This study supports a growing realisation (e.g. Debus & Chafer 1994) that Powerful Owls will defend their young against humans, contrary to earlier impressions (Fleay 1968, Schodde & Mason 1980). Fleay himself obtained indications, from the wild and captivity, that the owls may defend young. The strength of aggressive behaviour may vary with nest height and other circumstances. From my observation of 31 nesting events, it is apparent that the adults are more readily agitated when a low hollow (e.g. (10m above ground) is approached. It is likely that in the experience of Gibbons (1989) and Hollands (1991), extreme aggression by the adults may have been provoked by the excessive pressure of regular human presence near the low (10 m) nest.

The dietary data agree with previous studies, and support the view that in lowland areas where Ringtail Possums are common and Greater Gliders are scarce, the Ringtail is the predominant prey (e.g. Seebeck 1976, Tilley 1982, Hollands 1991, Chafer 1992, Debus & Chafer 1994, Lavazanian et al. 1994, Wilson et al. 1994). In the present study area with high Ringtail densities, the owls selectively concentrated on that presumably optimal (most rewarding) prey species, and apparently had no need to forage for ground prey as occurs elsewhere (e.g. Chafer 1992). For example, the Ferntree Gully owls apparently ignored a male Superb Lyrebil"d Menura novaehollandiae that regularly displayed beneath their roosts, although the Lyrebird is occasionally recorded as prey elsewhere (Chafer 1992).

The dietary records and foraging observations suggest that the owls in this study hunted mostly in the tree canopy and shrub stratum, and confirm that insects are taken by hawking (cf. Lavazanian et al. 1994). Nevertheless, young owls showed a willingness to forage or investigate prey on the ground. The suggestion that juvenile brushtail possums are taken from their mothers' backs (VanDyck & Gibbons 1980, Wilson et al. 1994) remains doubtful until witnessed, on the grounds that an adult possum with riding young would present a large, bulky and defensive target in contrast with an inexperienced juvenile separated from its mother.

Finally, the estimated home-range sizes (c. 300 ha) for two pairs are much smaller than previous estimates elsewhere, although the pairs in this study may have ranged over larger areas than detected. Previous estimates have cited home ranges of 800-1000 ha or more per pair (Fleay 1968, Seebeck 1976, Schodde & Mason 1980), although Davey (1993) estimated minima of 400-600 ha. The larger estimates have been seized upon and quoted in some circles, for use as conservation criteria (e.g. Possingham 1991), perhaps misleadingly. It is apparent that home-range size varies according to habitat quality and prey density, with small home ranges in productive habitat as at Olinda and Femtree Gully.

294

Conclusions

McNABB AUSTRALIAN

BIRD WATCHER

In parts of southern Victoria exemplified by this study, the Powerful Owl may be characterised as an inhabitant of highly productive habitat where it lives in small home ranges, preys on abundant Ringtail Possums, nests in massive old eucalypts in moist gullies where it achieves a high reproductive output, amid varied (multiaged) forest with abundant hollows and a complex understorey. Other studies demonstrate a much greater flexibility in habitat, diet and/or nest-site characteristics over the owl's range (e .g. Hollands 1991, Chafer 1992, Debus & Chafer 1994, Pavey et al. 1994, Pavey 1995). However, there appears to be a connection between habitat quality! home-range size, diet breadth and breeding success, with owls in less productive habitat occurring more sparsely and breeding less successfully on a more generalised diet.

This study suggests that the Powerful Owl's optimal habitat elements, fostering high breeding productivity, include high densities of massive old-growth trees in sheltered positions and orientations, with a choice of new hollows every 2-4 years, in traditional nesting gullies. Optimal hollows are in living trees sheltered by the canopy, and surrounded by neighbouring eucalypts and a shrub layer in which owlets can shelter. In the wider home range, optimal conditions are met by understorey elements and tree-hole densities that support high densities of preferred prey. Habitat management for Powerful Owl conservation should, therefore, maximise the longterm availability of optimal nest sites and high numbers of preferred prey. ·

The long-term survival of the Powerful Owl in Victoria will depend largely on management strategies that conserve old-growth forest patches in suitable gullies, and key habitat elements for prey species (hollows, shrub layer) across the wider landscape. The ancient eucalypts which provide nesting hollows for the owls and their major prey species should therefore be retained. Recruits for the next generation of hollow trees are also required, to replace the current generation of nest trees as they fall down; a continuous supply of all age classes is desirable (Ambrose 1982).

Acknowledgements

I thank the following people: Margaret McNabb (deceased) for 15 years of encouragement and patient support for this study; Don Saunders, ex Director, and Amiss Hieslers, ex Research Coordinator, Victorian National Parks, for their interest, support and relevant bird-banding and research permits; officers of the Australian Bird and Bat Banding Schemes. The following past and present Department of Conservation and Natural Resources (DCNR) researchers, officers, rangers and staff all supported the study in various ways: Barbara Baxter, Bill Burnett, Kevin Curran, Mike Cusack, Fiona Hamilton, John Lloyd, Richard Loyn, Bruce Makerith, Peter Menkhorst, David Newton, John Seebeck, Bob Seamer and Bob Warneke. Steve Craig (DCNR) climbed nest trees, provided reference material and made many helpful suggestions. David Middleton gave generously of both his veterinary and tree-climbing skills. Norm Carter, Peter Lewis, Des Quinn, Robert Taylor and Alan Webster (DCNR) shared their nest sites and/or data with me. Richard Zann trusted me with his copy of Ambrose's thesis. Tom Aumann and David Baker-Gabb taught me about banding raptors . I also thank Charles Compton, Susie Dewar-McNabb, Cyril Fawcett (deceased), Deirdre Harris, Kelvin McNabb, Peter Trusler and others I may have forgotten for their company and assistance in the field. Susie DewarMcNabb and Julia Hurley assisted with the statistics and stages of the draft. Stephen Debus gave expert assistance with a draft of this paper and two anonymous referees gave constructive comments. Last, but not least, I thank Attila Ur who first stimulated my interest in the Powerful Owl.

References Ambrose, G.J . (1982), An Ecological and Behavioural Study ofVertebrates Using Hollows in Eucalypt

Branches, Ph.D. thesis , LaTrobe Unjversity, Melbourne. Baker-Gabb, D.J. (1993), In~erirr~; List of'F{Ireatened Fauna in Victoria in 1993, Dept Conservation

& Natural Resources , V1ctona. .I · Chafer, C.J. (1992), 'Observations of the P@werful Owl Ninox strenua in the illawarra and Shoalhaven

Regions of New South Wales ', Aust. '/Jird Watcher 14, 289-300.


Cupper, J. & Cupper, L. (1981), Hawks in Focus, Jaclin, Mildura.

295

Davey, S.M. (1993), 'Notes on the habitat of four Australian owl species' , in Olsen, P.D. (Ed.), Australian Raptor Studies, 126-142, Australasian Raptor Assoc., RAOU, Melbourne.

Debus, S.J.S. (1995), 'Surveys of large forest owls in northern New South Wales: methodology, calling behaviour and owl responses', Corella 19, 38-50.

--& Chafer, C.J., (1994), 'The Powerful Owl Ninox strenua in New South Wales' , Aust. Birds 28 suppl., 21-38 .

Favaloro, N. (1946), 'Eggs of the Powerful Owl', Emu 46, 44-48. Fleay, D. (1944), 'Watching the Powerful Owl', Emu 44, 97-112. -- (1968), Nightwatchmen of Bush and Plain, Jacaranda, Brisbane. Gibbons, D. (1989), 'Nest defence by Powerful Owl', Aust. Bird Watcher 13, 58-59. Hollands, D. (1991), Birds of the Night, Reed, Sydney. Jonnson, G. (1995), 'Coal Creek 22 .2.95 [excursion report]', Qld Omithol. Soc. News/. 26(3), 4. Kavanagh, R.P. (1988), 'The impact of predation by the Powerful Owl, Ninox strenua, on a population

of the Greater Glider, Petauroides volans', Aust. J. Ecol. 13, 445-450. Lavazanian, E., Wallis, R. & Webster, A. (1994), 'Diet of Powerful Owls (Ninox strenua) living

near Melbourne, Victoria', Wild/. Res. 21, 643-646. McNabb, E.G. ( 1994a), 'The successful rehabilitation of two Powerful Owl fledglings ', Aust. Bird

Watcher 15, 287-297. -- (1994b), 'Predator calls and prey response' , Viet. Nat. 111, 190-195. Olsen, P.D., & Marples, T.G. (1993), 'Geographic variation in egg size, clutch size and date of

laying of Australia raptors (Falconiformes and Strigiformes )', Emu 93, 167-179. Pavey, C. R. (1995), 'Food of the Powerful Owl Ninox strenua in suburban Brisbane, Queensland',

Emu 95, 231-232. --, Smyth, A.K. & Mathieson, M.T. (1994), 'The breeding season diet of the Powerful Owl

at Brisbane, Queensland', Emu 94, 278-284. Possingham, H. (1991), 'Extinction, the role of chance', Habitat Australia 19(1), 32-33. Quinn, D. (1993), 'Nesting Powerful Owls', Australasian Raptor Assoc. News 14, 70-71. Schodde, R. & Mason, I.J. (1980), Nocturnal Birds of Australia, Lansdowne, Melbourne. Seebeck, J.H. (1976), 'The diet of the Powerful Owl Ninox strenua in western Victoria' , Emu 76,

167-169. Strahan, R. (Ed.) (1983), The Australian Museum Complete Book of Australian Mammals , Angus

& Robertson, Sydney. Tilley, S. (1982), 'The diet of the Powerful Owl, Ninox strenua, in Victoria', Aust. Wild/. Res. 9,

157-175. Traill, B.J. (1991), 'Box-ironbark forests: tree hollows, wildlife and management', in Lunney, D.

(Ed.), Conservation of Australia's Forest Fauna, 119-123, Roy. Zool. Soc. N.S.W., Sydney. -- (1993), 'The diet and movements of a pair of Powerful Owls Ninox strenua in dry eucalypt

forest', in Olsen, P.D., Australian Raptor Studies, 155-159, Australasian Raptor Assoc., RAOU, Melbourne.

VanDyck, S. & Gibbons, D. (1980), 'Tuan predation by Powerful Owls', Viet. Nat. 97, 58-63. Wheeler, W.R. (1972), Birds of the Dandenongs, Field Naturalists Club of Victoria, Melbourne. Wilson, J., Wallis, R. & Webster, A. (1994), 'Diet of the Powerful Owl Ninox strenua in Warrandyte

State Park, Victoria', Australasian Wild/. Manage. Soc. News/. 7(2), 22 [abstract] .

Received 20 December 1994, revised 4 January 1996 •

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