Contents of some Powerful Owl pellets from the Melbourne region.
The diet of the Powerful Owl Ninox strenua is relatively well studied, particularly in the Melbourne area. Nevertheless, the opportunity was taken to collect 111 pellets, or partial pellets, regurgitated from birds at six sites across the Greater Melbourne area between April 2009 and February 2015. As expected from previous and more detailed studies, analysis of these pellets showed that Common Ringtail Possum Pseudocheirus peregrinus was the most common prey, and occurred in 79 pellets. Two unexpected findings were Swamp Wallaby Wallabia bicolor remains found in one pellet collected on Bourkes Creek, near the township of Emerald, and Little Red Flying-fox Pteropus scapulatus remains in three owl pellets collected at a site on the lower Yarra River. (The Victorian Naturalist 134 (4), 2017, 101-103)
Keywords: Powerful Owl, Melbourne, pellet analysis, diet
The Powerful Owl Ninox strenua is the largest species of owl occurring in Australia. While the species has adapted to urbanisation in some cities--where there are unnaturally high populations of its preferred arboreal mammal prey, such as possums (e.g. McNabb 1996; Cooke 2000)--the Powerful Owl is considered to be Vulnerable in Victoria (DSE 2013) and is listed as Threatened under that state's Flora and Fauna Guarantee Act 1988 (DELWP 2017).
Urban landscapes provide abundant prey, and large introduced trees, such as Monterey Pine Pinus radiata, deciduous willows Salix spp. and English Elm Ulmus procera are frequently used as roost-trees (e.g. Cooke et al. 2002; McNabb and McNabb 2011). Owls in urban areas also may be habituated to higher levels of disturbance than conspecifics in more natural areas (Cooke 2000). In urban areas, Powerful Owls may be restricted to areas of dense vegetation for roost sites. Moreover, breeding opportunities are limited by the reduced availability of old-growth trees with suitably sized hollows (e.g. McNabb and Greenwood 2011).
Opportunistic collections of 50 regurgitated pellets or partial pellets from Powerful Owls were made at five Melbourne Water work sites between April 2009 and February 2015 (Table 1). An additional 61 regurgitated pellets were collected by JG from a roost site near Hastings on the Mornington Peninsula, 58 km southeast of the Melbourne city centre, during November 2012 (n=27) and October 2013 (n=34). Many pellets were fragmented but all were collected below trees where Powerful Owls were observed roosting.
All pellets and pellet fragments were analysed by HB. The length and diameter of 69 intact pellets were measured to the nearest millimetre. Pellets were softened in water then broken up to separate bones from hair. Bones were identified by comparison with reference material, and hairs were identified using the methods of Brunner and Coman (1974) and Brunner and Triggs (2002). Further confirmation was provided by Barbara Triggs (see Brunner and Triggs 2002; Triggs 2004).
The frequency of each prey taxon was calculated as the percentage of pellets in which they occurred.
Pellets measured, on average, 48.4 ([+ or -]18.12) mm in length (n=69, range 16-110 mm) and 24.6 ([+ or -]3.82) mm in diameter (n=69, range 15-33 mm).
The two most common prey species found in pellets were Common Ringtail Possum Pseudocheirus peregrinus, which occurred in 79 of 111 pellets, and Common Brushtail Trichosurus vulpecula, which occurred in 35 of 111 pellets (Table 2). There were, however, some unexpected findings. An entire forepaw of a Swamp Wallaby Wallabia bicolor was found in one pellet (Table 2) collected on Bourkes Creek, near the township of Emerald, approximately 50 km south-east of Melbourne. Little Red Flying-fox Pteropus scapulatus, an uncommon visitor to southern Victoria (Menkhorst 1995), was found in three owl pellets (Table 2) collected at a site on the lower Yarra River.
The diet of the Powerful Owl is well studied (e.g. Higgins 1999), particularly around Melbourne (McNabb 1996; Menkhorst et al. 2005; Cooke et al. 2006; Fitzsimons and Rose 2010), yet we have found no reports of Swamp Wallaby as prey in Victoria. The Swamp Wallaby remains were found in an isolated pellet collected at one site. This unusual observation is of interest as Powerful Owls generally feed on arboreal mammals and are believed to take most of their prey in trees (e.g. Kavanagh 2002). However, comparable macropods have been recorded previously as Powerful Owl prey in other areas. For example, Schulz (1997) found three lower jaw bones of Herbert's Rock-wallaby Petrogale herberti in Powerful Owl pellets and observed a female Powerful Owl clutching a sub-adult rock-wallaby at a roost in Moonlight State Forest, near Rockhampton, Queensland. Further, Kavanagh (2002) reported juvenile Red-necked Wallaby Macropus rufogriseus in a pellet from New South Wales, although he recognised that some of his 'unusual records may represent errors in pellet sample identification' (he gave no reason why). So we have no reason to believe Powerful Owls would not occasionally prey upon small Swamp Wallabies. Another example occurred in New South Wales where Powerful Owls were reported to feed on road-killed carrion (Braithwaite 1996), so it is possible our remains may have been from carrion even though Powerful Owls most often are believed to take arboreal prey (Kavanagh 2002).
Flying-foxes Pteropus spp. have been recorded as important prey of the owl in some areas, especially urban areas of Brisbane and Sydney (Pavey et al. 1994; Higgins 1999). So, although previous studies near Melbourne have seldom reported flying-fox as prey (e.g. Cooke et al. 2006), it is not surprising that Powerful Owls should prey upon flying-foxes in Melbourne (Menkhorst et al. 2005), where the Grey-headed Flying-fox Pteropus poliocephalus is now established and has a growing population.
It is interesting that although Grey-headed Flying-foxes were common in the Menkhorst et al (2005) study area, only one individual was found among 73 pellet samples. This is contrary to prey capture rates near other flying-fox camps (e.g. Pavey et al. 1994). The Little Red Flying-fox (mean 450 g) is somewhat smaller than the Grey-headed Flying-fox (mean 700 g; Menkhorst and Knight 2001) and has been reported flying within flocks of the larger species. The Powerful Owl's main prey comprises relatively larger species, such as Ringtail and Brushtail Possum rather than the Little Red Flying-fox, and it is therefore unlikely that the latter species is preferred or targeted. One explanation may be that the smaller species cannot fly as fast as its sympatric, larger cousin and may therefore have been taken in flight by an owl chasing a flock.
Although we have collected only a small number of Powerful Owl regurgitation pellets in an ad hoc and opportunistic manner, our findings suggest that future studies may reveal a continuously expanding range of prey of this opportunistic predator. All pellets were collected between September and April (spring to autumn) when the owls may have been supporting young. Our small sample size, collected over many months at a number of sites, prevents discussion of seasonal or spatial patterns in the diet of the Powerful Owl. However, we hope that the data we present here might contribute to future comprehensive analyses of the diet of this species in urban areas to determine the full range of prey taken and how these might vary with the duration and/or intensity of urbanisation.
We thank David Fisher and Russell Barrett of Melbourne Water for initiating this study and making it possible through their encouragement of Powerful Owl pellet collection. We thank Barbara Triggs for her contribution and two referees for their valuable input.
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Received 10 December 2015; accepted 18 May 2017
EG McNabb (1), H Brunner (2), WK Steele (3), J Gunn (4) and T Dyer (5)
(1) PO Box 135 Gembrook, Victoria 3783; (2) 9 Cooinda Court, Frankston, Victoria 3199;
(3) Melbourne Water, PO Box 4342, Melbourne, Victoria 3001; (4) 90 Besgrove St Rosebud, Victoria 3939;
(5) 12 Carol Street, Mornington, Victoria 3931
Table 1. Sites around Melbourne where 111 Powerful Owl pellets were collected, April 2009 to February 2015. Site Location Position A Wilsons Reserve, lower 37.7802[degrees] S, 145.0459[degrees] E Yarra River B Bourkes Creek, Emerald 37.9501[degrees] S, 145.4703[degrees] E C Brimbank Park. 37.7308[degrees]S, 144.8427[degrees] E Maribyrnong River D Stammer's property, 38.2450[degrees] S, 145.7474[degrees] E Lang Lang River E Near Hastings, 38.3107[degrees]S, 145.1692[degrees] E. Mornington Peninsula F Doongala Reserve, 37.8446[degrees] S, 145.3320[degrees] E Kilsyth Table 2. Summary of prey items found in 111 Powerful Owl pellets collected at sites around Melbourne, April 2009 to February 2015. (see Table 1 for site details) Site Prey Species Frequency in pellets ACDEF Common Ringtail Possum Pseudocheirus peregrinus 79 (71.2%) DEF Common Brushtail Possum Trichosurus vulpecula 35 (31.6%) B Swamp Wallaby Wallabia bicolor 1 (0.9%) A Little Red Flying-fox Pteropus scapulatus 3 (2.7%) A unidentified Flying-fox Pteropus spp. 1 (0.9%) DE Black Rat Rattus rattus 2 (1.8%) A Australian Magpie Cracticus tibicen 7 (6.3%) ACE Feathers, unidentifiable species 11 (9.9%) AC Insects (may include post-deposition additions) 10 (9.0%) C Plant material 1 (0.9%) D Snail shell 1 (0.9%)
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|Title Annotation:||Research Reports; Melbourne, Australia|
|Author:||McNabb, E.G.; Brunner, H.; Steele, W.K.; Gunn, J.; Dyer, T.|
|Publication:||The Victorian Naturalist|
|Date:||Aug 1, 2017|
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