Male combat in the Powerful Owl Ninox strenua

Australian Field Ornithology 2015, 32, 190–200

Summary. This paper describes an account of combat between two Powerful Owls Ninox strenua in the St George area, Sydney, NSW. The breeding pair of this territory had been followed as part of a monitoring program since July 2012. Daily photographic records from February to May 2014 provided recognition of individual birds by observing their distinctive chest patterns. Two possible interpretations of the incident were generated. Based on the identity of the aggressor and a male changeover in the territory, the combat incident was believed to be a contest between rival males.

Introduction

The Powerful Owl Ninox strenua has been extensively studied in a range of forest types (Debus & Chafer 1994; Higgins 1999), including modified landscapes (Cooke et al. 2002a,b; McNabb 2004; Isaac et al. 2013). Many behavioural studies have focussed on diet and hunting (Kavanagh 2002; Menkhorst et al. 2005; Fitzsimons & Rose 2010; Olsen et al. 2011), breeding success (Lavazanian et al. 1994; Pavey et al. 1994; McNabb 1996; Hogan & Cooke 2010) and roost preferences (Pavey & Smyth 1998; Webster et al. 1999; Menkhorst et al. 2005).

Territorialism is documented in many owl species, e.g. Flammulated Owl Psiloscops flammeolus (Marshall 1939), Eurasian Scops Owl Otus scops (Galeotti & Sacchi 2001), Tawny Owl Strix aluco (Galeotti & Pavan 1993; Appleby & Redpath 1997) and Little Owl Athene noctua (Finck 1990). Breeding pairs of Powerful Owls certainly establish territories, which are defended from conspecifics (Debus 1995; McNabb 1996; Soderquist & Gibbons 2007). Some authors have reported occurrences of fighting and chasing in other owl species (Desmond & Savidge 1996; Severinghaus 2000; Sunde & Bolstad 2004), but to the best of our knowledge, detailed accounts of these events have not been published. There have, however, been accounts of male duelling in the Southern Boobook Ninox novaeseelandiae (Olsen & Trost 1997).

This paper describes an incident of combat in the Powerful Owl in the St George area, southern Sydney, New South Wales. The regular monitoring of these Owls for a broader study provided knowledge of events leading up to this confrontation, and some observations of subsequent rivalry between the two birds. These additional insights aid in contextualising the combat incident.

Male combat in the Powerful Owl Ninox strenua

Matthew Mo1*, Peter Hayler2 and Antonia Hayler2

1New South Wales Department of Primary Industries, Elizabeth Macarthur Agricultural Institute, Woodbridge Road, Menangle NSW 2568, Australia2Choosypix Photography, Oatley NSW 2223, Australia. Email: [email protected]*Corresponding author. Email: [email protected]

Male Powerful Owls fighting 191

Background to Powerful Owl study in the St George area

The presence of Powerful Owls in urban Sydney has received increasing attention since the 1980s (Chafer 1992; Rose 1993; Kavanagh 2004; McAllan & Larkins 2005). Preliminary results from a current study confirm the presence of >70 breeding pairs in the Sydney metropolitan area (D. Bain unpubl. data), reaffirming the highest densities occurring in the northern suburbs (Kavanagh 2004). A long-term study of urban-dwelling Powerful Owls along the lower Georges River has identified at least eight pairs between Picnic Point and Hurstville Grove (C. Lloyd & MM unpubl. data). The nesting sites of these Owls fall within a total area of ~550 ha. All nests on the northern side of the river were located in small public recreational areas (<50 ha) within 50 m of the boundaries with residential development.

One of the male Powerful Owls described in this paper had a territory based in the Oatley–Hurstville Grove area, where he bred with the same mate for at least 2 years before the combat incident. Two main sites used by the pair have been identified: the nesting hollow in Moore Reserve (Figure 1, a), and an area in Poulton Park (Figure 1, b) used for roosting when not breeding. Moore Reserve, a 14.2-ha park in Oatley, is predominantly occupied by open parkland and a man-made wetland, although remnant vegetation has been retained along the boundaries. Poulton Park is a larger bushland tract in Hurstville Grove. The Owls were mainly sighted in the southern half of this site where there is a transition zone between disturbed dry sclerophyll forest and mangroves.

The breeding pair of Powerful Owls was first located in Moore Reserve in July 2012, and subsequently monitored during the breeding seasons from 2012 to the present (Mo & Waterhouse 2015). The nest-hollow used was in the trunk of a Smooth-barked Apple Angophora costata, and the Owls were consistently located during the day and at dusk in roost-trees within 100 m of this site (cf. Fleay 1944; Tilley 1982; Traill 1993). The 2013 monitoring period ended on 25 November, when the adults could no longer be located in Moore Reserve, and surveying of their regular roosts over a 7-day period yielded neither further sightings nor traces of excreted material or pellets.

Events leading up to combat incident

The breeding pair of Powerful Owls was rediscovered at a section of the mangroves in the southern portion of Poulton Park on 23 February 2014 by PH and AH. A juvenile from the previous year remained dependent and continued to demand food from the adults. The Owls were monitored each morning and dusk until 11 May. Until the juvenile dispersed sometime in mid April, all three birds were located on most days, and during this time their roosting was confined to a 0.2-ha area in Poulton Park.

The identities of the breeding pair were verified by comparing their chest plumage against photographic records. The female had relatively flat, horizontally aligned chevrons, giving the appearance of slightly curved barring (Figure 2a, p. 196); her sex was further confirmed by observing her behaviour in the subsequent

192 Australian Field Ornithology M. Mo et al.

breeding season. The male had deeper chevrons (as described by Higgins 1999), which were horizontally aligned on the right side of the chest and somewhat vertically aligned on the left side (Figure 2b). Extensive rusty brown tinges were also observable in the male.

On 27 April, an additional Powerful Owl (newcomer) was noticed for the first time. This Owl’s chevrons were vaguely defined in the top third of the chest and evenly distributed in the remainder of the chest area (Figure 2c). Following the combat incident, the newcomer began to attend the female, replacing the original male in this role. These observations were highly suggestive that the newcomer was a rival male.

Description of combat incident

The combat incident occurred on 3 May, during an observation period between 0845 and 1130 h. The weather was overcast with intermittent light rainfall. At 0900 h, the temperature was 12°C. More than 200 still photographs allowed further examination afterward. These were taken with a Canon EOS-1D X camera, using 1/250 shutter speed, aperture value 8.0, ISO speed 3200 and exposure compensation +2, with an EF 70–200-mm f/2.8L IS II USM +1.4× lens.

The newcomer was the first Powerful Owl to be located. In contrast with the typical lethargic demeanor of the Owls during the day, it was evidently agitated (Figure 3), restlessly turning its head and extending the neck. It had its wings splayed and feathers raised. The latter is thought to be a bluff tactic to avoid combat (Olsen & Olsen 1987). The original male was 4 m away on a higher perch, and maintained a continuous focus on the newcomer. Photographs of the original male taken at 0846 h show a feather dangling from his talon, which raises the suggestion of an earlier physical contact; however, the lack of bloodshed goes against this interpretation (C. Lloyd pers. comm.). The female was ~40 m from the disputing Owls, and did not move throughout the observation period.

Figure 1. The Powerful Owl territory concerned in this paper, with the two parklands labelled: (a) Moore Reserve, Oatley, (b) Poulton Park. The exact locations of the nest-hollow and roost-sites are not shown to reduce possible disturbance.


At 0847 h, a brief confrontation occurred whereby the original male Powerful Owl flew down to the newcomer’s perch (Figure 4) and extended his head and neck toward the newcomer. For the next 33 minutes, both Owls intermittently changed perching positions, relocating 1–2 m at a time, in silence. At 0920 h, the original male perched <2 m from the newcomer on a sloping branch (similar to Figure 4). From this time to 0926 h, the original male advanced a few centimetres at a time. The newcomer responded by edging farther up the branch, apparently trying to avoid confrontation. It anxiously rotated its head several times, as if examining possible escape routes. By 0953 h, the newcomer had stopped retreating and the Owls were within 30 cm of each other. Continuous eye contact between them was maintained for 3 minutes.

At 0956 h, the original male Powerful Owl appeared to lose interest momentarily and turn slightly away, but then edged closer. In response, the newcomer lifted a foot toward its opponent (Figure 5), which leapt upward and presented his talons toward the chest area of the newcomer (Figure 6). Both Owls spread their wings and fluttered. As the original male dropped, he missed his perch and descended to a lower branch (Figure 7). The confrontation lasted only 2 seconds. The original male flew over to the female at 1119 h (Figure 8) and remained there for the remainder of the observation period. The entire event occurred in silence.

The dusk observation period on the same day began at 1650 h. The original male could not be located and did not call. The newcomer and the female were found at different perch positions from those last observed, which suggests that further activity took place in the afternoon.

Aftermath and subsequent rivalryNear-nightly monitoring on dusk (1700–1830 h) was conducted in May and June. Before the combat incident, the original male mated with the female at least seven times between 28 April and 1 May. Each mating lasted for up to 3 seconds. Following the dispute, the female was seen accompanied only by the newcomer, which was observed to mate with her on 4 May at 1728 h.

The Powerful Owls were observed in Moore Reserve for the first time in the year on 12 May, with the female investigating the nest-hollow used in previous years (Mo & Waterhouse 2015). Until 26 May, the newcomer travelled from the roost-site in Poulton Park to the nest-site each evening to accompany the female. After this date, he was roosting near the nest-hollow.

The original male was rarely present following the combat incident, being observed at the nest-site on only four occasions. On 22 May, both males were present, neither holding prey. The newcomer called from the bottom of the gully, and the original male from the top of the gully. Although the original male’s calls began 10 minutes after the onset of the newcomer’s calls, they were more resonant. Nevertheless, the female emerged from the nest-hollow and joined the newcomer. Both males were present again the following night, with the newcomer clutching a Grey-headed Flying-fox Pteropus poliocephalus, but this evening the female did not appear during the observation period (1700–1840 h).


On 6 and 7 June, the original male was located at Moore Reserve, both times at least 30 m from the nest-hollow. These dates and one other evening were the only dates when the newcomer was not present at the nest-site. The original male called briefly (<5 min.), but did not receive a response from the female.

Interpretation of combat behaviour

Post-observation analysis of photographic records and the authors’ recognition of individual Powerful Owls from plumage chest patterns were useful for examining the underlying context of the incident described. The observation alone generated two possible interpretations:

1. The dispute was a case of male rivalry, as presented here.2. If the third Owl was the fledgling from the previous year, then the dispute

related to an adult Owl attempting to force away the juvenile to become independent.

The second interpretation was partly supported by the time of year of the observation. In New South Wales, juveniles may become independent as late as early May (Higgins 1999), and sometimes into the start of the subsequent breeding season (Debus & Chafer 1994; McNabb 1996). Despite this correlation, however, this interpretation does not account for the changeover of the male Owl attending the female.

Conclusions

Conflicts between Powerful Owls have mostly been documented in the form of vocal disputes (Fleay 1944; Schodde & Mason 1980; Kavanagh & Peake 1993). Physical aggression, described in this paper and by McNabb (1996), is less known. Given the large number of observational studies conducted, occurrences of physical aggression are probably scarce.

Similar-sized birds are included in the Powerful Owl’s diet (Evan 1986; Pavey 1994; McConnell 1997; Göth & Maloney 2012), and hence it can be deduced that conspecifics are easily capable of inflicting fatal injuries. McNabb et al. (2007) reported a possible case of a Powerful Owl being killed by a conspecific. There is also an account of a territorial female Southern Boobook killing another female Boobook (Olsen et al. 2002; Olsen 2011). The potential severity of physical contact may explain why agonistic behaviour consists predominantly of non-contact strategies, i.e. calling (Debus 1995; McNabb 1996), staring and feather-raising (Olsen & Olsen 1987). In birds, an assessment of the size of an opponent is important for predicting the probable outcome of a physical confrontation (Olsen & Olsen 1987; Appleby & Redpath 1997), hence restricting the need for combat to close matches. Certainly most of the incident described in this paper involved stand-off tactics, with only a momentary climax.

Apart from describing agonistic behaviour, these observations contribute to the study of the Powerful Owl in other aspects. The observation adds to the limited


information on the diurnal activity of this species, first described by Robinson (1968). Diurnal behaviour is characteristic of Barking Owls Ninox connivens (Debus & Rose 2003), and has also been recorded in the Common Barn Owl Tyto alba (Bunn 1972) and young Eurasian Eagle-Owls Bubo bubo (Penteriani et al. 2000). In addition, the rotation of males in the territory disproves a past belief that Powerful Owls mate for life (D. Fleay in Higgins 1999), which has also been challenged by Olsen (2011).

AcknowledgementsWe acknowledge David Waterhouse, who located the breeding pair of Powerful Owls in July 2012 and has actively followed the Owls’ progress since then. We also thank Chris Lloyd and Dr David Bain for their useful discussions. Useful comments by Jerry Olsen, Sue Trost and Ed McNabb improved the manuscript. The photographs used in this paper are the property of ChoosyPix Photography.

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Figure 2. The chest patterns of three Powerful Owls: (a) the resident female with flat, horizontally aligned chevrons; (b) the original male with deeper chevrons that are horizontally (right side of chest) and vertically (left side of chest) aligned; and (c) the newcomer, whose chevrons were irregular in the top third of the chest and evenly distributed in the remainder of the chest area. Photos: Peter and Antonia Hayler

Figure 3. The newcomer showing signs of agitation—feathers raised and wings splayed. Photo: Peter and Antonia Hayler


Figure 4. The original male (right) confronts the newcomer (left) by landing on the same perch. Photo: Peter and Antonia Hayler

Figure 5. The agitated newcomer (left) lifts a foot toward the advancing original male (right). Photo: Peter and Antonia Hayler


Figure 7. The original male (right) misses the perch in the aftermath of the leap, and drops toward a lower branch. Photo: Peter and Antonia Hayler

Figure 6. Both owls leap into the air, presenting their talons at the opponent’s chest area. Photo: Peter and Antonia Hayler


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Figure 8. The original male (left) joins the female (right) on a perch 40 m from the place of combat. Photo: Peter and Antonia Hayler


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Received 20 July 2014

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Male combat in the Powerful Owl Ninox strenua