Australian Field 2013, 30, 22–39

Does the relative abundance of large versus small arboreal determine sexual dimorphism in Powerful ?

Jerry Olsen1*, David Judge1, Susan Trost2 and A.B. Rose3

1Institute for Applied Ecology, University of Canberra, ACT 2601, 244 Wybalena Grove, Cook ACT 2614, Australia 3Associate, The Australian Museum, 6 College Street, Sydney NSW 2010, Australia (Present address: 61 Boundary Street, Forster NSW 2428, Australia) *Corresponding author. Email: [email protected]

Summary. The size of prey taken by Powerful Owls strenua, the relative densities (abundance) of small versus large arboreal marsupials in eucalypt , the lack of asymmetrical ears in Ninox owls, and the male’s habit of roosting on dead prey during the day may be clues to understanding ‘Normal’ Sexual Size Dimorphism in large Ninox species, the inverse of the ‘Reversed’ Sexual Dimorphism found in most owls, hawks and falcons.

Introduction In most and mammals, male–male competition has led to males being larger than females (‘Normal’ Sexual Size Dimorphism, NSD). However, in birds of prey the opposite is true: most exhibit ‘Reversed’ Sexual Dimorphism (RSD), with females larger than males. Over 20 hypotheses have been proposed and debated in the literature to explain RSD in raptors (Kruger 2005). In his analysis, Kruger said that these hypotheses fit into three broad theories: niche partitioning—sexes diverged in size to reduce intersexual competition for prey; role differentiation —females became larger to protect and efficiently incubate eggs, and/or males became smaller for more efficient foraging or territory defence; and behavioural —larger females dominate males, aid in the maintenance of the pair-bond and increase male food-provisioning, or larger females compete more effectively for males. Kruger collected data on 237 hawk (Accipitridae), 61 falcon (Falconidae) and 212 (Strigiformes) species, and determined a dimorphism index from the wing-length (mm) of males divided by wing-length of females. He performed comparative analyses, using both cross-taxa data and phylogenetically independent contrasts, to investigate potential correlates of RSD. Using a set of explanatory variables, covering morphology, life-history and ecology, he analysed 26 predictor values for hawks and falcons and 22 for owls, with RSD as the dependent variable. He found no evidence linking RSD to formation or size of eggs, laying or incubation, nor evidence linking RSD to females competing for males (sexual selection), nor any that males were sexually selected to become smaller in order to perform acrobatic flights. However, Kruger did find evidence for the intersexual- competition hypotheses, that males and females reduce competition for prey, and that males became smaller to forage for more agile and/or larger or rarer prey. : Sexual dimorphism and prey size 23

In his analysis of degrees of dimorphism in the world’s owls, Kruger found that Australia held the two extremes: the Sooty Owl Tyto tenebricosa exhibiting the most RSD and the Ninox rufa the least, because the Rufous Owl (and the Powerful Owl N. strenua) exhibit NSD, with males larger than females. Kruger did not discuss this anomaly. In large -catching falcons, for example the Prairie Falcon Falco mexicanus in North America, it is said that there is a ‘20% rule’: the adult falcon tends to catch birds that are, on average, ~20% of the falcon’s body weight (Boyce 1985). This rule broadly holds for falcons such as the Australian Peregrine Falcon F. peregrinus macropus (Olsen et al. 2008). The Powerful Owl, however, catches prey estimated by Kavanagh (2002) to be 50–100% of the owl’s weight. Some of the smaller Northern Hemisphere owls, such as the starling-sized Northern Pygmy- Owl Glaucidium gnoma, also take relatively large prey. For example, Pygmy-Owls tackle woodpeckers or thrushes and sometimes ride them flapping along the ground in the snow for as long as an hour until they subdue them (Johnsgard 2002). However, this behaviour is rare in raptors, and Pygmy-Owls take large prey mainly during winter, not during the breeding season; during breeding, they take mainly smaller birds of no more than 35 g, small mammals, and some (Johnsgard 2002). The Powerful Owl, Rufous Owl and Barking Owl Ninox connivens are exceptional among owls because they take large mammals as their breeding-season diet (Olsen 1990a,b; Debus 2009). Great Horned Owls Bubo virginianus are similar in weight to Powerful Owls, and take large prey, such as lagomorphs. However, the mean weight of prey taken by Great Horned Owls is not 25% of the weight of the owl, as has been suggested for Powerful Owls. In Colorado, USA, Marti (1974) found that Great Horned Owls took prey weighing from 1 g to ~3 kg (mean 177 g, which is heavier than the mean prey weight of 3 g for Burrowing Owls Athene cunicularia, 30 g for Long-eared Owls Asio otus or 46 g for Common Barn Owls Tyto alba breeding in the same area). In two other studies, Marti & Kochert (1995) calculated the Geometric Mean Prey Weight (GMPW) for Great Horned Owls as 76.0 and 44.5 g, which is higher than the mean calculated for Southern Boobooks (2.1 g) but lower than that for Powerful Owls (176.5 g) in the ACT (Olsen et al. 2011). Olsen (1990a,b) suggested that the Powerful Owl differs from other owls because it regularly takes proportionally large prey. Another large Australian owl, the Masked Owl Tyto novaehollandiae, takes small ground prey, from genera such as Rattus, Pseudomys and Antechinus and, where introduced species such as Rabbits Oryctolagus cuniculus and Black Rattus rattus are available, it also takes these terrestrial and scansorial mammals, although the Powerful Owl seldom does so (Kavanagh 2002; Bilney et al. 2006). Olsen (1990a,b) suggested that the Powerful Owl may be locked into a niche, capturing prey large in relation to the owl’s body size because females cannot grow larger if still nesting in tree- hollows in forests. This related to the habit of holding prey in large Ninox owls where, instead of caching prey as other owls do, Powerful, Rufous and Barking Owls often perch for the day on large dead prey. Pavey (2007) revisited these notions, finding that holding of prey was largely 24 Australian Field Ornithology J. Olsen et al. confined to breeding male Powerful Owls, and was seen most frequently during the incubation and brooding phases of breeding. His study did not clearly resolve the purpose of holding prey, but he suggested that food storage or territorial display may be its purpose. One general rule about abundance is that small are usually more abundant than large animals (Newton 1998; Krebs 2008). Ecologists use this linear relationship to predict expected abundance of an animal from its weight. For example, the Mala Lagorchestes hirsutus (~1250 g), a small Australian macropod, was estimated to have a population density of 2.4 individuals/km2. For North American mammals, Krebs (2008) calculated a density of 60 individuals/km2 for squirrels Tamiasciurus sp. (200 g) compared with only 0.5 individual/km2 for Elk Cervus elaphus (250 kg). It is not clear whether this relationship holds for arboreal marsupials in eucalypt forests in south-eastern Australia. Powerful Owls select large arboreal marsupials (such as Pseudocheirus peregrinus, Common Trichosurus vulpecula and Petauroides volans) as their main prey (Fleay 1968) more often than smaller ones (such as Squirrel Glider Petaurus norfolcensis, Yellow-bellied Glider P. australis, P. breviceps and Acrobates pygmaeus) (see e.g. Kavanagh 2002). If these smaller arboreal marsupials are more abundant, Powerful Owls are taking proportionally fewer of them compared with larger prey. However, if the opposite is true (i.e. larger arboreal marsupials are more abundant than smaller ones), Powerful Owls may take larger arboreal marsupials in proportion to the numbers of these in the wild, as claimed by Seebeck (1976), Tilley (1982), Lavazanian et al. (1994) and Cooke et al. (2006). To examine some of these questions and further explore the relationship between foraging behaviour and RSD, we conducted a study using five measures encompassing owl morphology and characteristics of the owl’s prey to investigate NSD and RSD in owls, with an emphasis on the Powerful Owl: (1) Dimorphism Index; (2) Geometric Mean Prey Weight (GMPW), and prey/predator ratio; (3) Standardised Food Niche Breadth (SFNB); (4) arboreal marsupials versus terrestrial prey in owls’ diets; and (5) relative abundances of large versus small arboreal marsupials in owls’ habitats. We compared Dimorphism Indices for 15 owl species from around the world; GMPW for 12 owl species, and mean weight of these to determine the prey/predator ratios; SFNB of five owl species; and diet composition (arboreal marsupials, terrestrial mammals, birds and invertebrates, following Kavanagh 2002) in four Australian species, to determine how the Powerful Owl differs from other forest owls. We also examined the proportions of large versus small possum and glider prey species reported in spotlighting surveys (six published, one unpublished) of arboreal marsupials in south-eastern Australian forests, to assess whether large or small arboreal marsupials were most often detected.

Methods Dimorphism Index Although Kruger (2005) used wing-length in his analysis of RSD in raptors, weight was Powerful Owl: Sexual dimorphism and prey size 25 used in the present study because wing-length can be misleading. Although Sunde et al. (2003) found in the Tawny Owl Strix aluco that females were larger than males in all body measurements, the differences were often slight compared with differences in weight: females were 16% heavier than would be expected from morphological differences, based on wing-length alone. Furthermore, in some species, males may have proportionally longer wings than females. For example, in the Burrowing Owl in Florida, USA, males have a slightly longer wing-length (166.3 vs 163.6 mm) but weigh slightly less (148.8 vs 149.7 g) than females (Haug et al. 1993). Using data from Higgins (1999), Johnsgard (2002), König & Weick (2008) and Debus (2009), Dimorphism Indices were calculated for 15 owl species, from the mean weight of males divided by the mean weight of females.

Geometric Mean Prey Weight (GMPW) and prey/predator ratio The role of prey size has been well studied for raptors elsewhere in the world (Nilsson 1984; Marti et al. 1993), and prey size is said to be an important factor in RSD; for example, RSD might have developed for hunting large and agile prey (Kruger 2005). One straightforward method of estimating raptor prey size is to calculate the mean prey weight. The frequencies of prey weights of most raptor species, however, do not follow normal distributions, and are usually skewed to one side of the mean. This poses a problem for using traditional arithmetic mean prey weights, since this statistic is not representative of the central tendency of skewed populations, and is therefore unreliable as an estimator of the overall prey size. In an attempt to fix this problem, Jaksić & Braker (1983) proposed the use of GMPW, obtained by summing the products of the number of prey items in each category multiplied by the loge-transformed weight for each prey item, and dividing the sum by the total number of prey individuals in all categories. This simple procedure has been used since as the standard method to estimate raptor prey size (see Marti 1987), but has seldom been used for Australian raptors (see McDonald et al. 2003). Consequently, we used GMPWs to compare the diet of the Powerful Owl with other owls. In this study, prey data were obtained from sources with enough dietary data to analyse (too few data were available for Rufous Owls, at the end of the continuum in Kruger’s 2005 dimorphism analysis). In addition, the prey/predator ratio for each species was calculated by dividing the GMPW by the mean weight of the predator (= average of male and female owl weights). For Kavanagh’s (2002) data, the weight of juvenile Common Brushtail Possums (570 g) was used because Kavanagh did not age these possums (R. Kavanagh pers. comm.), and most were probably juvenile and any adults taken were likely to be small. This assumption may have underestimated the mean prey size of Powerful Owls.

Standardised Food Niche Breadth (SFNB) To compare the dietary breadth (diversity) of the Powerful Owl with the North American Great (of similar weight), and the Australian forest-dwelling Masked Owl, Sooty Owl and smaller Southern Boobook Ninox novaeseelandiae, we used SFNB (Colwell & Futuyma 1971). Dietary diversity has two components: (1) richness (i.e. the number of prey species) and (2) evenness (i.e. how uniformly represented the various prey species are). A diet has high diversity (broader food niche) if many prey species are included in nearly equal numbers but low diversity (narrow food niche) if it contains few species in very different abundances (Marti 1987). To calculate SFNB, the following formula was used:

SFNB = (Bobs – Bmin) / (Bmax – Bmin) 2 where Bobs = 1 / ∑ pi ,

Bmax = total number of prey species,

Bmin = minimum number of prey species (= 1), and pi is the proportion of each prey species. 26 Australian Field Ornithology J. Olsen et al.

Arboreal marsupials versus terrestrial prey in diet Using categories based on those of Kavanagh (2002), and data from Pavey et al. (1994), Kavanagh (2002), Trost et al. (2008) and Olsen (2011a), we compared the proportions of arboreal marsupials (Common Ringtail Possum, Greater Glider, Sugar Glider), other arboreal mammals, terrestrial marsupials (dasyurids, bandicoots), , other ground mammals, other ground animals (reptiles, amphibians), birds, and invertebrates in the diets of large forest-dwelling owls (Powerful, Sooty and Masked Owls) and the smaller Southern Boobook.

Relative abundances of arboreal marsupials in the environment A common procedure for assessing the abundance of arboreal marsupials in areas frequented by Powerful, Masked or Sooty Owls is to use systematic spotlighting, transects, and/or listening for calls of arboreal marsupials (Kavanagh 1984; Lindenmayer et al. 1999; Wintle et al. 2005; Cooke et al. 2006). Several researchers, including Seebeck (1976), Tilley (1982), Lavazanian et al. (1994) and Cooke et al. (2006), argued that Powerful Owls took arboreal marsupials in proportion to the abundance of these in the owls’ habitat. Tilley (1982) and Cooke et al. (2006) confirmed this with spotlight data showing that large arboreal marsupials were more abundant than smaller ones in the areas where they studied Powerful Owls, and Powerful Owls took more large arboreal marsupials. We used published data from James (1980), Tilley (1982), Bennett et al. (1991), Kavanagh & Bamkin (1995), Lindenmayer et al. (1999) and Ward (2000) and unpublished data from spotlighting counts conducted by the ACT Parks and Wildlife Service in the Tidbinbilla Nature Reserve from 1974 to 2009 (M. Evans pers. comm.) to compare the relative abundances of small (average 10–600 g: Feathertail Glider, Sugar Glider, Yellow-bellied Glider) versus larger (average 900–4000 g: Common Ringtail Possum, Greater Glider, , Mountain Brushtail Possum Trichosurus caninus) arboreal marsupials in south-eastern Australian forests.

Results Dimorphism Index To place Powerful Owls in a context with some other owls, we compared weights and degrees and direction of dimorphism for six Australasian and 10 Northern Hemisphere owls (Table 1). Sooty Owls showed the most pronounced RSD and Burrowing Owls the least. Rufous and Powerful Owls went against the trend, with males larger than females (NSD).

Geometric Mean Prey Weight (GMPW) and prey/predator ratio Table 2 shows the Geometric Mean Prey Weight (GMPW), predator weight and prey/predator ratio for four Australasian and eight Northern Hemisphere owl species. Powerful Owls showed a higher prey/predator ratio than other species, including the (which is of similar weight). However, Sooty Owls also took proportionally large prey.

Standardised Food Niche Breadth (SFNB) Table 3 shows the SFNB for four Australasian forest owls and the Northern Hemisphere Great Horned Owl. SFNB was broadest for Southern Boobooks and Great Horned Owls and narrowest for Powerful Owls in the ACT (Olsen et al. 2011). In Kavanagh’s (2002) data, Powerful Owls had broader niches than did Powerful Owl: Sexual dimorphism and prey size 27

Table 1. Approximate weights (g) of male and female owls, and their rank using a simple Dimorphism Index (DI = average weight of male divided by average weight of female, expressed as a percentage). * = Powerful Owl and Rufous Owl males are larger than females. See text for sources.

Species Male Female DI

Sooty Owl Tyto tenebricosa 650 1120 58.0 Rufous Owl Ninox rufa (n. Australia) 1450 950 152.6* Rufous Owl (e. Qld) 950 660 143.9* Masked Owl Tyto novaehollandiae (n. Australia) 480 700 68.6 Strix nebulosa 900 1200 75.0 Great Horned Owl Bubo virginianus 1300 1700 76.5 Tawny Owl Strix aluco 470 580 81.0 Snowy Owl Bubo scandiaca 1650 2000 82.5 Powerful Owl Ninox strenua 1500 1250 120.0* Northern Pygmy-Owl Glaucidium gnoma 63 73 86.3 Northern Hawk Owl Surnia ulula 300 345 87.0 Southern Boobook Ninox novaeseelandiae 280 320 87.5 (Australian mainland) Spotted Owl Strix occidentalis 570 650 87.7 Elf Owl Micrathene whitneyi 40 45 88.9 Southern Boobook (New Zealand) 177 193 91.7 Little Owl Athene noctua 172 175 98.3 Burrowing Owl Athene cunicularia 149 150 99.3

Masked or Sooty Owls. Overall, SFNB did not significantly separate the Powerful Owl from the other two large forest owls (Masked and Sooty Owls).

Arboreal marsupials versus terrestrial prey in diet Figure 1 shows the number of arboreal marsupials compared with other prey (other arboreal mammals, terrestrial mammals, and other animals, birds and invertebrates) in the diets of four Australasian forest owls, based on the categories used by Kavanagh (2002). Powerful Owls took proportionally more arboreal marsupials and fewer terrestrial prey species than did Sooty Owls, Masked Owls or Southern Boobooks. Great Horned Owls took no arboreal mammals (such as squirrels), but took more terrestrial rodents and birds than did similarly sized Powerful Owls. The Ninox species took more invertebrates (often flying insects taken in mid air: see Olsen 2012) than did other owls.

Relative abundances of arboreal marsupials Table 4 shows the relative abundances (as indicated from detection rates) of large 28 Australian Field Ornithology J. Olsen et al. 1200 Powerful Owl, NSW (Kavanagh 2002) n = 1672 1000

800

600

400

200

0 80 Powerful Owl, Qld (Pavey et al. 1994) n = 209Inv. CRtP GrGl SuGl OAM Dasy Band OGA Birds 70 Rodents 60 50 40 30

20 10 0 50 Powerful Owl, ACT (Olsen et al. 2011) n = 62Inv. GRtP GrGl SuGl OAM Dasy Band OGA Birds 45 Rodents 40 35 30 25 Number of prey items 20 15 10 5 0 450 Sooty Owl, NSW (Kavanagh 2002) n = 1466Inv. CRtP GrGl SuGl OAM Dasy Band OGA Birds 400 Rodents 350 300 250 200 150 100 50 0 CRtp GrGl SuGl OAM Dasy. Band. Rod. OGA Birds Inv. Inv. CRtP GrGl SuGl OAM PreyDasy speciesBand OGA Birds Figure 1. Prey species in the diets of large forestRodents owls, Southern Boobook and Great Horned Owl: arboreal marsupials [Common Ringtail Possum (CRtP), Greater Glider (GrGl), Sugar Glider (SuGl)], other arboreal mammals (OAM), terrestrial marsupials [dasyurids (Dasy.), bandicoots (Band.)], rodents (Rod.), other ground mammals (OGM), other ground animals (OGA) (amphibians and reptiles), birds, and invertebrates (Inv.). Sources of data are shown. Powerful Owl: Sexual dimorphism and prey size 29 1200 90 Masked Owl, NSW (Kavanagh 2002) n = 175 1000 80 800 70 600 60 400 50 200 40

0 30 80 Inv. CRtP GrGl SuGl OAM Dasy Band OGA Birds 20 70 Rodents 10 60 50 0 40 350 Inv. CRtP GrGl SouthernSuGl OAM Boobook,Dasy Band ACT (Trost OGAet al. 2008) n = 496 30 Birds 300 Rodents 20 10 250 0 200 Inv. GRtP GrGl SuGl OAM Dasy Band OGA Birds Rodents 150

100

50 Number of prey items

0 CRtp GrGl SuGl OAM Dasy. Band. Rod. OGA Birds Inv. Inv. CRtP GrGl SuGl OAM Dasy Band OGA Birds 2000 Great Horned Owl (MurphyRodents 1997) n = 2900 1800 1600 1400 1200 1000 800 600 400 200 0 AborealOAM RodentsRod. OGMOGM OGA Birds Inv.Inv. mammals Prey species Figure 1. Prey species in the diets of large forest owls, Southern Boobook and Great Horned Owl: arboreal marsupials [Common Ringtail Possum (CRtP), Greater Glider (GrGl), Sugar Glider (SuGl)], other arboreal mammals (OAM), terrestrial marsupials [dasyurids (Dasy.), bandicoots (Band.)], rodents (Rod.), other ground mammals (OGM), other ground animals (OGA) (amphibians and reptiles), birds, and invertebrates (Inv.). Sources of data are shown. 30 Australian Field Ornithology J. Olsen et al.

Table 2. Geometric mean prey weight (GMPW), weight of owl (average of male and female), and prey/predator ratio (GMPW/predator weight) of some owls in Australia and North America. Source: 1 = Kavanagh (2002), 2 = Pavey et al. (1994), 3 = Marti et al. (1993), 4 = Murphy (1997), 5 = Tishechkin (1997) and 6 = Olsen et al. (2006). Weights of owls are taken from Higgins (1999), Johnsgard (2002) and Debus (2009).

Predator Source GMPW (g) Predator Prey/predator weight (g) ratio Powerful Owl 1 386.7 1375 0.281 Ninox strenua 2 323.22 1375 0.235 Great Horned Owl 3 44.0 1500 0.029 Bubo virginianus 4 43.41 1500 0.029

Great Grey Owl 5 25.3 1030 0.025 Strix nebulosa Masked Owl 1 74.66 590 0.127 Tyto novaehollandiae Sooty Owl 1 185.13 885 0.209 Tyto tenebricosa Common Barn Owl 3 32.5 521 0.062 Tyto alba Northern Hawk Owl 6 24.9 295 0.084 Surnia ulula Southern Boobook 6 2.11 283 0.008 Ninox novaeseelandiae Long-eared Owl 3 26.3 254 0.104 Asio otus Burrowing Owl 3 2.3 170 0.014 Athene cunicularia Western Screech Owl 3 18.0 191 0.094 Megascops kennicottii Northern Saw-whet Owl 3 18.7 170 0.110 Aegolius acadicus

Table 3. Standardised Food Niche Breadth (SFNB) of five owl species. Species Source SFNB

Powerful Owl Olsen et al. (2011) 0.115 Masked Owl Kavanagh (2002) 0.179 Sooty Owl Kavanagh (2002) 0.206 Powerful Owl Pavey et al. (1994) 0.235 Powerful Owl Kavanagh (2002) 0.260 Great Horned Owl Murphy (1997) 0.284 Southern Boobook Trost et al. (2008) 0.325 Powerful Owl: Sexual dimorphism and prey size 31

Table 4. Spotlight counts for eight species of arboreal marsupials in seven studies: percentage contribution of each species to the total number of animals detected in each study (number of prey items in parentheses). Approximate weight of each prey species based on Menkhorst & Knight (2001) and van Dyck & Strahan (2008). Studies: 1 = Kavanagh & Bamkin (1995) (unlogged areas only), 2 = Bennett et al. (1991), 3 = Ward (2000), 4 = Tilley (1982), 5 = James (1980), 6 = unpubl. data for ACT, 1974–2009 (M. Evans pers. comm.), and 7 = Lindenmayer et al. (1999).

Species (weight, g) Study Total 1 2 3 4 5 6 7

Mountain Brushtail 3.9 6.2 0 0 0 0 8.5 3.3 Possum (11) (87) (4) (102) (4000) Common Brushtail 4.2 34 4.9 21.6 11.3 28 21.3 26.5 Possum (12) (477) (3) (126) (6) (182) (10) (816) (3500) Greater Glider 24.6 30.8 60.7 0 22.6 48.1 31.9 28.6 (1300) (70) (433) (37) (12) (313) (15) (880) Common Ringtail 4.6 20.9 31.1 76 30.2 20.4 23.4 30.1 Possum (13) (294) (19) (442) (16) (133) (11) (928) (900) Yellow-bellied 23.6 1.4 0 0 0 0 0 2.8 Glider (67) (20) (87) (575) Squirrel Glider 0 1.5 0 0 0 0 2.1 0.7 (230) (21) (1) (22) Sugar Glider 38.7 4.2 1.6 2.2 32.1 2.9 6.4 7.2 (125) (110) (59) (1) (13) (17) (19) (3) (222) Feathertail Glider 0.3 0.9 1.6 0.1 3.8 0.6 6.4 0.8 (12) (1) (13) (1) (1) (2) (4) (3) (25)

Total 284 1404 61 582 53 651 47 3082

(average 900–4000 g) versus small (average ~10–600 g) arboreal marsupials in south-eastern Australian forests, based on six published and one unpublished study. Larger species were detected more often than smaller species in all studies except that of Kavanagh & Bamkin (1995), the difference being highly significant (χ2 = 1822.5, P <0.0001).

Discussion Dimorphism Index In the ranking in Table 1, Sooty, Masked and Great Grey Owls Strix nebulosa exhibited the greatest RSD, and Burrowing and Little Owls Athene noctua the least. Owls that hunt in forest, such as Great Grey, Masked and Sooty Owls, often have greater dimorphism than species that hunt over open ground, such as Burrowing 32 Australian Field Ornithology J. Olsen et al. or Little Owls, perhaps because owls living in forest need agility to avoid collisions (Olsen 1989) or because the distribution of prey species is skewed towards smaller animals in forest. Male raptors are said to do much of the hunting during the breeding season, and their smaller size gives them more agility compared with females, and greater energy efficiency when carrying prey over long distances (Newton 1979). Athene species, such as Burrowing and Little Owls, may be less dimorphic because they are insectivorous, do not carry prey over long distances, and live mostly in open country, whereas forest raptors may be more dimorphic because they need to be energetically more efficient to hunt and carry prey around and in between trees (Olsen 2012). NSD in Powerful and Rufous Owls (Table 1), as well as Barking Owls, is unique amongst owls.

Geometric Mean Prey Weight (GMPW) and prey/predator ratio The prey/predator ratios in Table 2, calculated from the GMPW and the average weights of the male and female for each species, indicate that Powerful Owls take proportionally heavier prey than any other species, ~28% of the owl’s weight in Kavanagh’s (2002) large sample. They take proportionally heavier prey than does the Great Horned Owl, a powerful eagle-owl of similar weight. Common Ringtail Possums, a major prey item of the Powerful Owl, weigh 700–1100 g (average 900 g: Table 4). The Greater Glider, another important prey species, weighs ~900–1700 g (average 1300 g: Table 4). That is, two of the main prey animals of the Powerful Owl in certain parts of its range can weigh almost as much as the owl itself. Perhaps the large size of male Powerful Owls gives them greater impact when they hit large prey, and the power to lift heavy prey, and also reduces the number of dangerous hunting trips (see p. 35).

Standardised Food Niche Breadth (SFNB) The SFNB was broadest for Southern Boobooks and Great Horned Owls, and narrowest for Powerful Owls in the small ACT sample (Table 3). Overall, SFNB did not significantly separate the Powerful Owl from the other two forest owls, the Masked and Sooty Owls. The Southern Boobook and Great Horned Owl, with their broad SFNBs, also have proportionally larger distributions across and the Americas, respectively, and they are found across more habitat types than are Masked, Sooty and Powerful Owls (Olsen 2011a), all of which are limited by forests and forest prey species. Masked Owls can range out into woodlands and open areas near forest, but do so much less than Boobooks do (S. Debus pers. comm.).

Arboreal marsupials versus terrestrial prey in the diet Proportions of arboreal marsupials versus terrestrial mammals, birds, and invertebrates differed between owl species (Figure 1). The Powerful Owl’s reliance on arboreal marsupials indicates that this species evolved to fill a particular niche. The Tyto species take many terrestrial prey species, whereas the Powerful Owl Powerful Owl: Sexual dimorphism and prey size 33 does not (see Kavanagh 2002). Ninox owls hunt mostly by sight, and lack the broad heads and asymmetrical ears that Tyto owls use to triangulate ground prey by sound (Burton 1973). Although Southern Boobooks do hunt terrestrial prey, they often do so in open areas such as along roadsides or tracks (S. Debus, J. Olsen & S. Trost unpubl. data), and Boobooks may have developed certain acute hearing abilities without asymmetrical ears (Swanson & Sanderson 1999).

Relative abundances of arboreal marsupials in the environment In the surveys of arboreal marsupials (Table 4), large marsupials such as the Common Ringtail Possum, brushtail possums and Greater Glider were generally more common than smaller species such as the Sugar Glider and Feathertail Glider. By far the most widely employed field technique for detecting arboreal mammals in Australia is spotlighting (Lindenmayer et al. 2001), in which reflected eyeshine and/or the body shape of an animal is detected by a hand-held high-powered light beam (e.g. Bennett et al. 1991). However, there have been questions recently about how accurately spotlight surveys reflect arboreal mammal abundances. The detectability of arboreal marsupials by spotlighting can depend on weather and other conditions (Goldingay & Kavanagh 1988; Davey 1990), but this seems to pertain to both large and small animals. Lindenmayer et al. (2001) fitted radio- collars to 20 Greater Gliders, and found that their ability to detect radio-tagged animals was low: nine successful detections by spotlighting from 35 known animals in patches (26%), or 8% chance of detecting a collared animal in a single spotlighting pass. That is, spotlighting underestimated the numbers of these large arboreal marsupials. Goldingay & Sharpe (2004) said that spotlighting was at least as effective as trapping for estimating numbers of Squirrel Gliders, detecting, on average, 25% of animals present. To study detectability, additional variables (such as nest-boxes) were introduced in some studies. Ward (2000) found that spotlighting provided important information on the biology of Feathertail Gliders, but checking nest-boxes provided better estimates. However, as most areas with Powerful Owls lack nest-boxes, it is not clear how relevant this finding is. Wintle et al. (2005) measured ‘false absences’, i.e. how likely it was to detect an animal thought to be present. Of four arboreal marsupials—Sugar, Yellow-bellied, and Greater Gliders, and Common Ringtail Possum—from single visits to an area, they ranked Sugar Glider as the species most likely to be detected, followed in order by Yellow-bellied Glider, Greater Glider and Common Ringtail Possum. The evidence so far does not seem to show that in spotlight surveys the large arboreal marsupials preferred by Powerful Owls (e.g. Common Ringtail Possums) are over- estimated compared with smaller ones (e.g. Sugar Gliders). That is, we did not find evidence to refute the claims of Seebeck (1976), Tilley (1982), Lavazanian et al. (1994) and Cooke et al. (2006) that Powerful Owls in their studies took larger arboreal marsupials in proportion to the abundance of these in the environment, though this aspect needs further study. Even if large arboreal marsupials did occur at densities similar to those of smaller ones, this does not fit the usual models of large mammals occurring at lower densities than small ones. Lindenmayer (2002) noted that body weight and metabolic rate are strongly related: larger animals usually have a higher absolute 34 Australian Field Ornithology J. Olsen et al. metabolic rate and therefore require more food, typically correlated with a larger home-range and lower density. However, these patterns did not apply to gliders. In his analysis, Yellow-bellied Gliders had by far the largest home-ranges and lowest densities (0.02–0.28 individual/ha), but were intermediate in size compared with other marsupial gliders (see Table 4). The reason for this appeared to be mainly diet. The largest glider, the Greater Glider, which does not have to travel far to find suitable food (eucalypt leaves), had densities of 0.01–5 individuals/ ha, higher than most densities of Yellow-bellied Gliders but similar to Sugar Gliders (0.01–6.1 individuals/ha) even though the latter are about one-tenth of the weight of Greater Gliders. Greater Gliders are slow-moving and have relatively slow metabolism. Yellow-bellied and Sugar Gliders need trees with suitable sap- flows, and these trees are relatively uncommon; if Greater and Sugar Gliders do occur at similar densities, it would be more energy-efficient for Powerful Owls to search for, kill, and carry one larger Greater Glider than ten smaller Sugar Gliders (R. Bilney pers. comm.). Compared with most owls, the Powerful Owl has a proportionally small head, as do forest-dwelling goshawks and sparrowhawks Accipiter spp., and this may relate to its accipiter-like pursuit of quarry in the forest canopy. With its small head, it lacks the spacing and asymmetrical ears found in owls with broad heads that allow triangulation of sounds and facilitate frequent predation on rodents and terrestrial marsupials. Though Common Ringtail Possums are found to be the most common prey regularly taken in many dietary studies of Powerful Owls (Kavanagh 2002), one would assume that breeding pairs of Powerful Owls cannot survive on these possums alone and would need to take other abundant prey. For example, if Common Ringtail Possums are an optimal prey size for Powerful and Sooty Owls, Powerful Owls would take more prey the size of Ringtail Possums and larger, but Sooty Owls could exploit more prey the size of Ringtail Possums and smaller because, with asymmetric ears, the latter can successfully take forest rodents and other terrestrial vertebrates.

Roosting on prey Powerful, Barking and Rufous Owls exhibit NSD and roost on dead prey during the day. This sets them apart from all owls of genera other than Ninox. An eagle- owl, such as the Great Horned Owl of North and South America, usually caches large uneaten prey under a bush or hides it in a tree-hollow, instead of draping it over a limb in broad daylight where other predators could see the prey and take it (Johnsgard 2002). Olsen (1990a) argued that Powerful Owls perch on prey during the day because they catch prey proportionally large for the size of the owl, and could not evolve larger body size because of the limiting size of tree-hollows in the forest. This difference may be because male Powerful Owls control prey from their (smaller) mates (Olsen 1990b; McNabb 1996), and display this prey to females. In other raptors with RSD, such as the Southern Boobook and Peregrine Falcon, males deliver prey often to females, from before laying and through the nestling stage. In species where males compete for females (Olsen 1994; Kruger 2005), this practice may contribute to the males keeping females on the territory, so females do not Powerful Owl: Sexual dimorphism and prey size 35 change territories during the breeding season (Olsen 2011a,b). Powerful Owls would have difficulty making repeated deliveries to females, because it takesa night or two to collect each prey item (Hollands 1991, 2008). However, they can make controlled feedings to females by hanging onto large prey and not giving the whole prey to females. This also reduces the number of hunting forays and reduces the risks of accident through collision (Olsen 1989), and males could be the main source of food storage for the pair, in contrast with those raptors having RSD (Olsen 2012). Another advantage could be that larger raptors can go longer without food than smaller ones can, so larger males are advantaged. Owl species with females larger than males, such as the Great Horned Owl, cannot do this because females simply bully food away from males. Southern Boobooks, though more closely related to the Powerful Owl than to eagle-owls, have females larger than males, and Boobooks normally cache prey instead of perching on it for the day (Olsen 2011a,b). Also, it may simply be difficult for Powerful Owls to stow something as large as a Greater Glider or Common Brushtail Possum in a hollow, though they sometimes do this (pers. obs.). In Australia, all four Ninox owls—Southern Boobook, Rufous, Powerful and Barking Owls—take flying or arboreal insects near their nests, and carry these invertebrates one at a time to nestlings; they travel farther to capture and carry vertebrate prey (Hollands 2008; Olsen 2011a,b; Stanton 2011). This behaviour sets them apart from other owls around the world that weigh ~300g+, none of which is known to regularly take aerial insects and deliver these to nestlings (J. Marks & I. Zuberogoitia pers. comm.). It may be that curved beaks and talons incur benefits to insectivorous owls, allowing them to easily dismantle invertebrates with hard exoskeletons [such as beetles (Coleoptera)], but incur costs when transporting such prey, so owls hunt invertebrates close to the nest and do not transport them long distances (Olsen 2012). This aspect could advantage smaller female Powerful, Barking and Rufous Owls. Kruger (2005) found evidence in his analysis that male and female reduce competition for prey, and especially that males are smaller to forage for more agile and/or larger or rarer prey. Olsen (2012) argued that, in many raptor species, males take larger prey than females do. This includes the four Australian Ninox owls, one of which exhibits RSD, and three of which exhibit NSD. Both RSD and NSD may relate to males taking larger prey, but species such as the Powerful Owl take proportionally larger prey than do species with RSD. Kruger (2005) also considered the defence potential of prey animals. Olsen (2012) argued that, unlike invertebrate prey, larger vertebrate prey such as birds or mammals tactically use cover to evade capture. This puts the pursuing raptor at increased risk of collision or injury. A raptor with even a slightly injured wing, caused by hitting a branch or colliding with flocking birds, will be unable to forage efficiently and may die (Olsen 1990c). In contrast with the claim that female raptors need to avoid collision because of their developing eggs (Walter 1979), males in particular need to reduce the risk of collision because they hunt more vertebrate prey than do females. Males need to reduce the number of dangerous hunting forays (Olsen 1990b). 36 Australian Field Ornithology J. Olsen et al.

A related factor worth considering is that the glider species taken by the Powerful Owl and other Ninox species defend themselves (Lindenmayer 2002). Even though Sugar Gliders are smaller than Black Rats (125 vs 180 g: Menkhorst & Knight 2001), Southern Boobooks take many Black Rats but seldom take Sugar Gliders (Figure 1), apparently because these gliders fight back. Common Brushtail Possums do the same, and are sometimes attacked by Powerful Owls as enemies, not food (Olsen 2011a, 2012). Greater Gliders and Phascolarctos cinereus are possibly the slowest-moving animals in the forest, and raptors may require strength instead of agility to take them. The defence potential of these possums and gliders in eucalypt forest may be important in the NSD question.

Conclusions The size of prey taken by Powerful Owls, the proportion of large versus small arboreal marsupials in eucalypt forests, the owls’ habit of roosting on dead prey during the day, and the defence potential of prey may be clues to understanding NSD in Ninox owls. Southern Boobooks appear to have good hearing and take some ground prey; they may be able to pinpoint prey without asymmetrical ears (as they have broad heads). We need more data on the diet of the Rufous Owl, as this species may exhibit even greater NSD than the Powerful Owl. Also, we need better surveys of forest arboreal marsupials to determine if smaller species are actually more common or occur at densities similar to larger species and, if not, why Australian eucalypt canopies differ from other habitats in having large mammal species more common than small species (cf. Newton 1998; Krebs 2008). Owls with NSD are associated with Australasian eucalypt forests, which are dominated by a single genus, . Pure eucalypt browsers, vegetarians (e.g. Greater Glider) might be more common than species that are partly insectivorous and eat a lot of plant/ exudates, nectar etc. (e.g. Squirrel Glider), because these larger marsupials can feed on the toxic leaves of eucalypts. They may need such large body size to tolerate the doses of toxins ingested (S. Debus pers. comm.). The defence potential of prey species that use cover to avoid capture, versus prey species that are larger and fight back, also needs further investigation.

Acknowledgements Particular thanks go to Rohan Bilney, Julia Hurley, Oliver Kruger, John Mendelsohn, Cheryl Dykstra, Paul McDonald, Iñigo Zuberogoitia, Rod Kavanagh and Stephen Debus for helpful suggestions. Thanks go also to many people for field assistance, including Anthony Overs, Michael Lenz, Susan Robertson, Geoffrey Dabb and Mark Clayton from the Canberra Ornithologists Group; also Dalice Trost, Mark Osgood and the ACT Parks and Conservation Service, especially O. Arman, S. Taylor, C. Gould, M. Evans for supplying data on ACT arboreal marsupial frequencies, D. Fletcher, B. McNamara, M. Muranyi, P. Higginbotham, K. Boyd, M. Doepel, S. Tozer, and D. Rosso, who gave assistance in the field and access to the ACT Nature Parks. D. Drynan of the Australian Bird and Bat Banding Schemes and V. Hurley generously supplied colour-bands for the Southern Boobook studies, and B. Mannan, A. Georges, J. Hone, C. Krebs, S. Sarre, T. Dennis, J. Jolly, N. Mooney, D. Bird and L. Boyd provided helpful discussion. This study was carried out with permission from the University of Canberra Animal Ethics Committee #97/5. Powerful Owl: Sexual dimorphism and prey size 37

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Received 29 August 2011