186 AUSTRALIAN FIELD ORNITHOLOGY 2006, 23, 186–191 Prey Partitioning and Behaviour of Breeding Masked Owls Tyto novaehollandiae on the Central Coast of New South Wales MICHAEL TODD 187 Excelsior Parade, Toronto, New South Wales 2283 (Email: [email protected]) Summary Masked Owls Tyto novaehollandiae were observed at a nest that successfully fledged two owlets near Newcastle, central coastal New South Wales, over winter 2005. The nest was situated in bushland supporting native mammal species. The female Owl was observed to bring Bush Rats Rattus fuscipes (n = 3) and a Petaurus glider to the nest. In contrast, the male brought only smaller prey including the Brown Antechinus Antechinus stuartii (n = 3) and House Mouse Mus domesticus (n = 3). The combined feeding rate in the second half of the nestling period, when the female Owl was hunting, was 1.2 items/h (n = 13.1 h) in the evenings. Introduction Although the diet of the Masked Owl Tyto novaehollandiae has been examined in several areas of mainland Australia and Tasmania, most of these studies have been of regurgitated pellets rather than direct observations of captured prey (summarised by Higgins 1999; later studies by Kavanagh 2002 and McNabb et al. 2003). The Masked Owl exhibits strong reversed sexual size dimorphism (female 30% heavier than male: Higgins 1999), especially in the Tasmanian subspecies T.n. castanops in which the sexes take different-sized prey (prey of male averages 97 g, of female 261 g). The larger female Owl takes prey as large as European Rabbits Oryctolagus cuniculus (Mooney 1993). There are limited observations of such prey partitioning on the Australian mainland. From the hunting behaviour and perches of a pair of Masked Owls, and the mammals observed at those sites, Debus (1990) speculated that the male was hunting rats Rattus sp. and the female was hunting Rabbits. Debus & Rose (1994) reported that the stomachs of three males contained mice (mostly House Mice Mus domesticus) and a Black Rat Rattus rattus, and those of four females contained probable Mus, Long-nosed Bandicoot Perameles nasuta, Rattus sp. and Rabbit. Debus (1997) found that a radio-tracked female ate a Black Rat and probably Rabbits, and a male ate House Mice. The diet of an Owl family early in the nestling phase, when the male was probably doing all the hunting, consisted mostly of small prey (Antechinus spp. and a few Bush Rats Rattus fuscipes). Antechinuses, plus Sugar Gliders Petaurus breviceps, many Bush Rats and a large bird (probable Pied Currawong Strepera graculina), were taken later in the nestling phase when the female was probably also hunting (Kavanagh 1996). This paper reports prey partitioning by a pair of Masked Owls in forest in coastal New South Wales, as determined by observations of the male and female delivering prey to large nestlings and fledglings. Notes on behavioural aspects of this breeding event, other than those reported elsewhere (Todd 2006), are included. Study site and methods The study site was at Awaba (33°01′S, 151°33′E) on the north-western edge of Lake Macquarie, VOL. 23 (4) DECEMBER 2006Prey Partitioning of Breeding Masked Owls 187 on the Central Coast of New South Wales. The Owls’ nest was situated in a large Smooth-barked Apple Angophora costata in a well-vegetated gully, in open forest on Crown land near Awaba State Forest. The nest-tree had a protrusion on the trunk ~20 m above ground, housing the entrance to a hollow (see Todd 2006). The area has been logged in the past, and several powerline easements and many vehicle tracks traverse the area, creating a heterogeneous forest; parts are dominated by a shrubby understorey, and others are grassy. Two adult Masked Owls attending an active nest were observed between 28 May and 20 July 2005. The first juvenile had fledged by 2 July; the second was downy and peering from the entrance on 5 July, and had fledged by 9 July (i.e. fledged on 7 July ±1 day). Extrapolating from a nestling period of 2 months (Debus 1997), hatching occurred around early May and egg-laying in early April. The owlets were thus ~3 weeks old when observations of prey deliveries commenced on 28 May. A total of 13.1 hours was spent watching the nest. The Owls were photographed using a spotlight and SLR camera with a 420-mm telephoto lens and flash. The birds were accustomed to the light and flash over time, by limiting observation times to 2 hours every few days until they appeared unconcerned by the spotlight. On only one occasion did an adult Owl appear to not enter the nest as a result of the spotlight being turned on; consequently, the light was turned off and the night’s observations abandoned to avoid further disturbance. On many occasions during the observations, the adults carrying prey landed on a bare branch of an adjacent tree opposite the nest-tree, before flying to the nest. On some of these occasions, photographs taken were of sufficient quality to enable identification of the prey. By the end of the study period two fledglings were successfully raised and could fly. They were often fed away from the vicinity of the nest- tree, thus making identification of items difficult. After fledging, when fully grown and no longer downy, the juveniles were readily identifiable because they were both of the dark morph, whereas both parents were of the intermediate (white- breasted) morph (see Todd 2006). Results On 28 May only the male Owl was observed to bring prey to the nest. From 4 June, i.e. when the owlets were a month old, the female also brought prey to the nest; the following data on feeding rates and identified prey refer to the second half of the nestling period, when both sexes were hunting. Observations of prey brought to the nest suggested prey partitioning, by size, between the Owl sexes (Table 1). The male brought small mammals, the Brown Antechinus Antechinus stuartii (~28 g) (Plate 27) and House Mouse (~17 g), to the nest (mean or median prey weights from Strahan 1995). The female brought Bush Rats (average weight 216 g), and a small Petaurus glider that would have weighed between 100 and 250 g. Both the Sugar Glider (average weight 128 g) and Squirrel Glider P. norfolcensis (average weight 230 g) are common in the local area and district. Of the 10 identified prey items (Table 1), six were terrestrial rodents, three scansorial marsupials, and one an arboreal marsupial (prey habits from Strahan 1995). Table 1 Observations of prey brought to offspring by two adult Masked Owls at Awaba, NSW, June–July 2005. M, F = prey brought by male and female Owl, respectively. Prey species M F Brown Antechinus Antechinus stuartii 30 Glider Petaurus sp. 0 1 House Mouse Mus domesticus 30 Bush Rat Rattus fuscipes 03 Unidentified 5 1 Total 11 5 AUSTRALIAN 188TODD FIELD ORNITHOLOGY Adult Masked Owls, Awaba, NSW, July 2005 Above: Male with Brown Antechinus; below: female. Plate 27 Photos: Michael Todd VOL. 23 (4) DECEMBER 2006Prey Partitioning of Breeding Masked Owls 189 Juvenile Masked Owls, Awaba, NSW, July 2005 Plate 28 Photo: Michael Todd The parents brought prey to the nest at an average rate of one item every 49 minutes (16 items in 13.1 hours, or 1.2 items per hour of observation). The male brought 0.8 item per hour, and the female brought 0.4 item per hour, from the stage of the nestling phase at which she started hunting. Observations were not conducted during the early hours of the morning, and it is not known whether this capture rate was consistent throughout the night. After both owlets had fledged, they grabbed each other’s claws in play, and allopreened each other’s facial ruff (see also Todd 2006). In the first week of the post-fledging period (while the second owlet was still in the nest) the adults continued to bring prey to the nest. From week 2 the juveniles sometimes ventured to trees across the gully from the nest. Over the first 3 weeks the adults brought prey to branches of the nest-tree, where the juveniles returned from neighbouring trees to receive it. A month after fledging, the juveniles were still in the nest gully, and 5 weeks after fledging one was perched on a horizontal branch ~8 m above the ground, apparently watching for rats. During the post-fledging period, the female parent sometimes screeched loudly when a human observer was in the nest area. In April 2006 the adult Owls were attending the nest-tree, and were observed to copulate once. In May the male flew to the nest-hole while the female was inside, and calls suggestive of copulation emanated from within. The male then reappeared at the entrance and called. A Powerful Owl Ninox strenua started hooting ~100 m down the gully, and the male Masked Owl stopped calling and stared intently in that direction until the Powerful Owl had stopped calling. The male Masked Owl then flew off, and the female also emerged and flew off. Despite these initial signs of possible nesting, the Masked Owls were not detected in the area in June. However, a Sooty Owl Tyto tenebricosa began calling regularly in the area, in rather dry habitat for that species. In July the adult Masked AUSTRALIAN 190TODD FIELD ORNITHOLOGY Owls were found, but there appeared to be no current nesting activity. The decayed remains of a Masked Owl were also found in the area in July: apparently a dark bird (i.e. not the adult female), and possibly a juvenile or immature on the basis of an apparently pointed outermost primary (cf.