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Does the Relative Abundance of Large Versus Small Arboreal Marsupials Determine Sexual Dimorphism in Powerful Owls?

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Does the Relative Abundance of Large Versus Small Arboreal Marsupials Determine Sexual Dimorphism in Powerful Owls? Australian Field Ornithology 2013, 30, 22–39 Does the relative abundance of large versus small arboreal marsupials determine sexual dimorphism in Powerful Owls? Jerry Olsen1*, David Judge1, Susan Trost2 and A.B. Rose3 1Institute for Applied Ecology, University of Canberra, ACT 2601, Australia 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 Ninox strenua, the relative densities (abundance) of small versus large arboreal marsupials in eucalypt forests, 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 birds 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 owl (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. Powerful Owl: 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 Rufous Owl 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 bird-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 insects (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 forest 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 Rats 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 animal abundance is that small animals 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 Common Ringtail Possum Pseudocheirus peregrinus, Common Brushtail Possum Trichosurus vulpecula and Greater Glider Petauroides volans) as their main prey (Fleay 1968) more often than smaller ones (such as Squirrel Glider Petaurus norfolcensis, Yellow-bellied Glider P. australis, Sugar Glider P. breviceps and Feathertail Glider 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.
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