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A powerful Owl disperses into town and uses an artificial nest-box
Abstract
A female Powerful Owl Ninox strenua was banded as a nestling at Lysterfield, Victoria, in 2001. She was found in 2007, 19 km south-west of her natal site, in the Melbourne suburb of Blackburn, nesting in an artificial hollow, where subsequently one young was successfully raised. The nest-box was provided by a local community group after becoming aware that the Owls were attempting to breed at a site where suitable hollows are scarce. Nest-box design is presented, and issues relating to the use of nest-boxes are outlined.
... Existing habitat creation measures, such as nest boxes, are inadequate. Only once has a nest box supported owl breeding, and even then, one of the two chicks died [6]. Existing nest-box designs fail to replicate the affordances of the natural structures. ...
... Existing nest-box designs fail to replicate the affordances of the natural structures. As a result, rectilinear nest boxes can entrap their inhabitants [6]. Their geometry and materials overheat [7]. ...
... This analysis converts point clouds output by scanning into voxels according to criteria important for humans and owls. Criteria known to influence owls' selection of nests include orientation [19,20] (Fig. 6 left) and height [6,21] (Fig. 6 middle left). Criteria important for the viability of an installation include placement at crotch junctions ( Fig. 6 middle right) and in areas that are structurally sound (Fig. 6 right). ...
Anthropogenic degradation of the environment is pervasive and expanding. Human construction activities destroy or damage habitats of nonhuman lifeforms. In many cases, artificial replacement habitats become necessary. However, designing for the needs and preferences of nonhuman lifeforms is challenging. Established workflows for this type of designing do not exist. This paper hypothesises that a multi-scale modelling approach can support inclusive, more-than-human design. The case-study project tests this approach by applying computational modelling to the design of prosthetic habitats for the powerful owl (Ninox strenua). The proposed approach simulates owls’ perception of the city based on scientific evidence. The tools include algorithmic mapping, 3D-scanning, generative modelling, digital fabrication and augmented-reality assembly. Outcomes establish techniques for urban-scale planning, site selection, tree-scale fitting, and nest-scale form-making. The findings demonstrate that computational modelling can (1) inform more-than-human design and (2) guide scientific data collection for more inclusive ecosystem management.
... What evidence therefore exists for powerful owls occupying areas where they are unable to breed? Powerful owls have been documented as inhabiting and persisting in many urban areas throughout our study area over a long period of time (McNabb and Greenwood, 2011). A clear example of such occupancy occurs in the Royal Botanical Gardens of Melbourne, a large park in close proximity to the city of Melbourne. ...
... dueting vocalisations, bleating, preening and roosting together) and copulation was observed in 2005. This culminated unsuccessfully with the female actively attempting (but failing) to enter and enlarge a cavity in a nearby tree (McNabb and Greenwood, 2011). In 2006 several nest boxes were erected within the pair's territory in an attempt to facilitate breeding. ...
... The use of nest boxes for the management of species occurs worldwide, with uptake of the nest box by the target species highly variable (Rohrbaugh and Yahner, 1997;Fargallo et al., 2001). Poor uptake can be reduced by providing nest boxes as similar as possible to natural tree cavities within cavity limited environments (Rohrbaugh and Yahner, 1997;Fargallo et al., 2001) but have already proven successful for a pair of powerful owls in suburban Melbourne (McNabb and Greenwood, 2011). ...
... Possibly because of such difficulties, the Owl was once thought to be a strict habitat specialist of extensive old-growth forest (Fleay 1968;Traill 1993). Since the late 1980s, many breeding pairs have been found persisting in developed areas and the urban interface, including Melbourne (Cooke et al. 2002a(Cooke et al. ,b, 2006Menkhorst et al. 2005;Isaac et al. 2008;Fitzsimons & Rose 2010;Hogan & Cooke 2010;McNabb & Greenwood 2011), Brisbane (Pavey et al. 1994;Pavey 1995;Pavey & Smyth 1998) and the Greater Sydney area (Kavanagh 1988;Chafer 1992;Rose 1993;McAllan & Larkins 2005). In Sydney, Powerful Owls have established territories close to the central business district (Kavanagh 2004). ...
... Pellet analysis revealed at least two additional vertebrate species that did not appear in direct observations (Table 1). Both are known prey items-Grey-headed Flying-fox Pteropus poliocephalus (Pavey et al. 1994;Menkhorst et al. 2005;McNabb & Greenwood 2011), and Rainbow Lorikeet Trichoglossus haematodus (Pavey et al. 1994). The former are sporadically captured by another breeding pair with a territory ~3 km away (C. ...
This paper extends previous observations of behavioural development in Powerful Owl Ninox strenua fledglings. The study combines a near-daily visual monitoring program on a pair of owlets in Oatley, suburban Sydney, New South Wales, with corresponding pellet analysis. The fledglings were initially fed on possums, fruit-bats, birds and insects, and first demonstrated independence by disassembling carcasses by themselves. By October, they apparently mimicked the adults’ strategy for capturing insects, and began to chase birds and bats. Behaviours thought to be part of honing their hunting skills—including tearing and ferrying strips of bark, foliage-snatching, and swooping at animals on the ground—were recorded. Such actions intensified during a period when the adults were mostly absent in November and December.
... Evaluating the management of reusable nest boxes is necessary during research studies. The service life should be extended for as long as possible, and regular inspection (better in the nonbreeding season) is needed to confirm nest box integrity (McNabb and Greenwood, 2011) and sanitation. Clean nest boxes, particularly deep ones, can attract birds, reduce parasite infection, and prevent nest hunting. ...
Artificial nest boxes are placed to attract birds to nest and breed in a specific location, and they are widely used in avian ecology research and in the attraction of insectivorous birds. There is evidence that artificial nest boxes can adversely affect breeding fitness but no great focus has been placed on this issue by researchers. Therefore, we retrieved 321 research papers regarding artificial nest boxes published from 2003 to 2022 and used the ‘Biblioshiny’ program to extract and integrate keywords; we then summarized the adverse effects of artificial nest boxes on avian breeding success. The studies highlighted many drawbacks and misuses in the designing and placement of nest boxes; furthermore, bird attraction was decreased by their inappropriate selection, thus reducing breeding success. Regarding nest box production, there were shortcomings in the construction material, color, smell, and structural design of the boxes used. Nest boxes were also placed at inappropriate densities, locations, orientations, heights, and managed incorrectly. Finally, we propose suggestions for more efficient and safer artificial nest boxes for future use in avian ecology research and bird conservation.
... Existing provisions for owls include nest boxes, hollows from felled trees, and even repurposed waste bins (Fig. 5). To date, there is only one observation of a successful powerful-owl breeding in a nest box-and only one of the two chicks survived [20]. The rectilinear forms of plywood nest boxes do not match the material and geometric complexity of the natural structures they aim to replicate. ...
Urbanization severely reduces opportunities for nonhuman habitation and undermines nonhuman subjectivities, aesthetic experiences, behaviors, traditions, and cultures. In response, humans need to reimagine cities as places for interspecies cohabitation. In this article, a team of architects and ecologists demonstrates that such reimagination depends on the cultural behaviors of multiple species. The authors illustrate the implications of this dependence by designing and discussing nesting structures for the powerful owl (Ninox strenua). The project shows that prosthetic habitats can serve as useful provocation for thinking about interspecies cultures. The authors use this work to propose productive avenues for further research.
... The project had an ethics approval from the Faculty of Science's Animal Ethics Committee at the University of Melbourne (#1914736).We used attributes of successful nesting sites, including tree cavities and arboreal termite mounds, to inform the resulting generative designs. In tree cavities, these features include a landing lip, a feeding platform, a climbable exit and an interior suitable for scratching(McNabb & Greenwood, 2011). Termite nests were also relevant as precedents because, unlike decayinduced cavities, they can occur on sufficiently large but young trees. ...
The decline of critical habitat structures, such as large old trees, is a global environmental challenge. The cavities that occur in these trees provide shelter and nesting sites for many species but can take centuries to develop. Artificial cavities, including nest boxes and carved logs, offer an increasingly important conservation response. However, current methods of designing, manufacturing and deploying such habitats have constraints that limit innovation, feasibility and effectiveness. In response, this article aims to provide new and broadly useable methods that can improve the design of habitat structures for cavity‐dependent animals.
To address the shortcomings of existing methods, we develop an approach that uses computer‐aided design techniques of generative and parametric modelling to produce structures that satisfy stakeholder needs, computer‐aided manufacturing techniques of 3D printing and augmented‐reality assembly to build functional prototypes, and computer‐assisted techniques of laser scanning and data‐driven design to support installation, monitoring and iterative improvement of designs. We demonstrate this approach through a case‐study project that designs and instals habitat structures for the powerful owl Ninox strenua , a cavity‐dependent and threatened bird.
Through a comparison with existing methods, our pilot study shows that computer‐aided design and manufacturing can provide novel and useful approaches to develop artificial habitat‐structures. Computer‐aided design finds geometries that approximate the complex characteristics of natural tree cavities and automatically produces new versions to suit diverse sites or species. Computer‐aided manufacturing integrates materials that match the performance of naturally occurring habitat structures and facilitates the assembly of complex geometries by non‐experts. Computer‐assisted techniques produce precisely fitting and easy‐to‐instal designs, which support gradual improvement through ongoing prototyping and evaluation.
These capabilities highlight how advanced design techniques can improve aspects of artificial habitat‐structures through geometric innovation, novel construction techniques and iterative exploration. Significantly, computational approaches can result in designs that can perform well, are easy to construct and instal and are applicable in many situations. Our reusable workflow can aid in the tasks of practical conservation and support ecological research by effectively negotiating the needs of both humans and target species.
... For example, the introduction of artificially constructed nest sites in urban areas invite birds that do not typically dwell in cities to recognise new opportunities for habitation. 36 To accept this invitation, birds need to learn to recognise and accept artificial nesting sites, be tolerant of noise and the presence of humans and their devices, modify their hunting strategies, adjust their diets and modify how they socialise. The next section sketches a conception of design that can benefit from integrating more-than-human cultures. ...
The Anthropocene is a proposed geological epoch marking humanity's alteration of the Earth: its rock structure, environments, atmosphere. The Cambridge Companion to Literature and the Anthropocene offers the most comprehensive survey yet of how literature can address the social, cultural, and philosophical questions posed by the Anthropocene. This volume addresses the old and new literary forms - from novels, plays, poetry, and essays to exciting and evolving genres such as 'cli-fi', experimental poetry, interspecies design, gaming, weird, ecotopian and petro-fiction, and 'new' nature writing. Studies range from the United States to India, from Palestine to Scotland, while addressing numerous global signifiers or consequences of the Anthropocene: catastrophe, extinction, 'fossil capital', warming, politics, ethics, interspecies relations, deep time, and Earth. This unique Companion offers a compelling account of how to read literature through the Anthropocene and of how literature might yet help us imagine a better world.
... 14 Although only a single event, the recent successful breeding of a powerful owl pair in a nest box in an urban area provides hope for the potential use of nest boxes as a conservation management strategy to alleviate the limited cavity resources for powerful owls in urban environments. 48 New technologies such as GPS tracking, spatial modeling, and genetics have certainly improved our understanding of this species. There are, however, still many basic ecological questions about these urban birds that we cannot answer. ...
Once thought to live only in large forested areas, the powerful owl (Ninox strenua), Australia’s largest and most iconic of owls (figure 11.1), surprisingly is now turning up frequently in the cities of eastern Australia. Powerful owls require ample prey and large tree cavities for nest sites; how this top-order predator is able to survive in human-dominated landscapes is an important question for conservation and the focus of ongoing research. The powerful owl is endemic to Australia, resident in the three eastern mainland states and the Australian Capital Territory, and classified nationally as “rare.”2,3 First described by Gould in 1838, powerful owls are an unusual raptor in that they do not exhibit reversed sexual size dimorphism, the prevalent trait among raptors in which females are larger than males. For reasons still not understood, male powerful owls grow to a height of 65 cm and weigh up to 1,700 g, compared to females, which grow to a height of 54 cm and weigh up to 1,308 g.1
The biodiversity crisis is exacerbated by a growing human population modifying nearly three‐quarters of the Earth's land surface area for anthropogenic uses. Habitat loss and modification represent the largest threat to biodiversity and finding ways to offset species decline has been a significant undertaking for conservation. Landscape planning and conservation strategies can enhance habitat suitability for biodiversity in human‐modified landscapes. Artificial habitat structures such as artificial reefs, nest boxes, chainsaw hollows, artificial burrows, and artificial hibernacula have all been successfully implemented to improve species survival in human‐modified and fragmented landscapes. As the global shift towards renewable energy sources continues to rise, the development of photovoltaic systems is growing exponentially. Large‐scale renewable projects, such as photovoltaic solar farms have large space requirements and thus have the potential to displace local wildlife. We discuss the feasibility of ‘conservoltaic systems’ – photovoltaic systems that incorporate elements tailored specifically to enhance wildlife habitat suitability and species conservation. Artificial habitat structures can potentially lessen the impacts of industrial development (e.g., photovoltaic solar farms) through strategic landscape planning and an understanding of local biodiversity requirements to facilitate recolonization.
Artificial refuges are human‐made structures that aim to create safe places for animals to breed, hibernate, or take shelter in lieu of natural refuges. Artificial refuges are used across the globe to mitigate the impacts of a variety of threats on wildlife, such as habitat loss and degradation. However, there is little understanding of the science underpinning artificial refuges, and what comprises best practice for artificial refuge design and implementation for wildlife conservation. We address this gap by undertaking a systematic review of the current state of artificial refuge research for the conservation of wildlife. We identified 224 studies of artificial refuges being implemented in the field to conserve wildlife species. The current literature on artificial refuges is dominated by studies of arboreal species, primarily birds and bats. Threatening processes addressed by artificial refuges were biological resource use (26%), invasive or problematic species (20%), and agriculture (15%), yet few studies examined artificial refuges specifically for threatened (Vulnerable, Endangered, or Critically Endangered) species (7%). Studies often reported the characteristics of artificial refuges (i.e. refuge size, construction materials; 87%) and surrounding vegetation (35%), but fewer studies measured the thermal properties of artificial refuges (18%), predator activity (17%), or food availability (3%). Almost all studies measured occupancy of the artificial refuges by target species (98%), and over half measured breeding activity (54%), whereas fewer included more detailed measures of fitness, such as breeding productivity (34%) or animal body condition (4%). Evaluating the benefits and impacts of artificial refuges requires sound experimental design, but only 39% of studies compared artificial refuges to experimental controls, and only 10% of studies used a before‐after‐control‐impact (BACI) design. As a consequence, few studies of artificial refuges can determine their overall effect on individuals or populations. We outline a series of key steps in the design, implementation, and monitoring of artificial refuges that are required to avoid perverse outcomes and maximise the chances of achieving conservation objectives. This review highlights a clear need for increased rigour in studies of artificial refuges if they are to play an important role in wildlife conservation.
The effects of habitat fragmentation on native wildlife can vary depending on the
type of land use occurring in the matrix between remaining habitat fragments. I used
Australian boobooks (Ninox boobook) in Western Australia to investigate interactions
between matrix type and four different potential threatening processes: secondary
poisoning by anticoagulant rodenticides (ARs); limitation of juvenile dispersal and impacts
on spatial genetic structure; breeding site availability; and infection by the parasite
Toxoplasma gondii.
I also conducted a literature review on the use and regulation of ARs in Australia and
published accounts of non-target impacts in order to contextualise exposure patterns
observed in boobooks. The review revealed records of confirmed or suspected poisoning
across 37 vertebrate species in Australia. World literature relating to AR exposure in
reptiles suggests that they may be less susceptible to AR poisoning than birds and mammals.
This relative resistance may create unevaluated risks for wildlife and humans in Australia
where reptiles are more abundant than in cooler regions where AR exposure has been
studied in greater depth.
I analysed AR residues in boobook livers across multiple habitat types. Second
generation anticoagulant rodenticides were detected in 72.6% of individuals sampled. Total
AR concentration correlated positively with the proportion of urban land use within an area
approximately the size of a boobook’s home range centred on the point where the sample
was collected. ARs originating in urban habitat probably pose a substantial threat to
boobooks and other predatory wildlife species.
No spatial genetic structure was evident in boobooks across habitat types. I
observed one individual dispersing at least 26km from its natal home range across urban
habitat. The apparent permeability of anthropogenically altered landscapes probably
explains the lack of spatial genetic structure and is likely related to the observed ability of
boobooks to use resources in both urban and agricultural matrices.
Boobooks did not appear to be limited by the availability of suitable nesting sites in
urban or agricultural landscapes. Occupancy did not change significantly over the duration
of the study in remnants provided with artificial nest boxes in either landscape type. However, in one instance, boobooks successfully used a nest box located in an urban
bushland. Nest boxes may be a useful management tool in highly-altered areas where
natural hollows are unavailable.
Toxoplasma gondii seropositivity in boobooks did not vary significantly by landscape
type but was more prevalent in individuals sampled during cooler wetter times of year. Risk
of exposure due to greater cat abundance in urban and agricultural landscapes may be
offset by creation of environmental conditions less favourable to the survival of T. gondii
oocysts in soil.
Taken together, this body of research demonstrates variation in relationships
between different types of habitat fragmentation and threatening processes related to
fragmentation. This research also raises questions about how habitat fragmentation is
discussed and studied in the context of species which are capable of making extensive use
of matrix habitat. I recommend greater consideration of the concept of “usable space”
when studying fragmentation impacts in habitat generalists.
Mobbing is an anti-predator strategy in which prey animals, notably birds and mammals, aggravate a
potential predator to either distract or drive them from the vicinity. The Powerful Owl Ninox strenua
is a large forest owl endemic to eastern Australia that preys mainly on arboreal mammals and birds.
We identified records of 30 species of birds and one mammal known to mob the Powerful Owl from
scientific literature and unpublished studies. In our study in southern Sydney, Powerful Owls were
most frequently mobbed by Noisy Miners Manorina melanocephala and Pied Currawongs Strepera
graculina, followed by Grey Butcherbirds Cracticus torquatus and Australian Magpies Cracticus tibicen. We
observed mobbing by three species of bird and one mammal that were not previously recorded as
mobbing species, including agonistic responses by a Tawny Frogmouth Podargus strigoides and Common
Brushtail Possum Trichosurus vulpecula during the time owls were active.
The diet of a single Powerful Owl Ninox strenua that roosted for over six months in a botanical garden close to the centre of Melbourne, Victoria, was investigated by analysis of regurgitated pellets. Two arboreal marsupials that are abundant in Melbourne parks and gardens comprised 52% of prey items: the Common Ringtail Possum Pseudocheirus peregrinus and Common Brushtail Possum Ti-ichosurus vulpecula. A further 31% of prey items were contributed by introduced rats, predominantly the scansorial Black Rat Rattus ratlus. A large camp of Grey-headed Flying-foxes Pteropus poliocephalus shared the garden with the Powerful Owl, but that species was not a favoured food item. The scarcity of eutherian mammals in previous dietary studies of the Powerful Owl, combined with the high incidence of introduced rats in the diet of this Owl, is further evidence that the Powerful Owl can be flexible in its dietary selection, favouring whatever suitably sized arboreal mammals are most available in its immediate environment.
The Powerful Owl Ninox strenua is now regularly reported in the Sydney area, New South Wales, though there were few records before the 1970s. Questions have arisen as to whether the population has grown in number or whether the species was previously overlooked. Historical records of the Owl for the Sydney area and changes in its local abundance are considered. Evidence for an increased population in parts of Sydney is given.
We describe conflict between a breeding pair of Southern Boobooks Ninox novaeseelandiae and several Common Brushtail Possums Trichosurus vulpecula that culminated in a possum entering the Boobook's nest-hollow and eating most of a clutch of eggs. however, one egg hatched, although a possum subsequently caused the young Boobook to fledge prematurely. After the juvenile could fly well, the Boobook family vacated the hollow-rich nesting patch for a foraging area that lacked hollows, where the juvenile reached independence. In the following year, the Boobooks nested again in the same tree but lost two clutches (in different hollows) to possums, which then occupied one of the hollows, before finally fledging two young from a third hollow (in a different tree) ~2 months later than normal. This hollow had been used earlier in the season by Sulphur-crested Cockatoos Cacatua galerita, which harassed the incubating Boobook and tried to enter the hollow. Competition for increasingly scarce hollows, by some species which are already abundant or are becoming more so, may be a factor in the recent decline of Boobooks.
The Powerful Owl is an elusive species inhabiting the forests of mainland eastern Australia. Obtaining crucial information on aspects of their breeding behaviour and dispersal has proven extremely difficult, even though other aspects of their ecology are well studied. Here we use molecular methods to investigate the breeding behaviour and dispersal of the Powerful Owl in two different habitats: highly fragmented forest along the urban fringe and continuous forest. DNA profiles of Powerful Owls were obtained predominately from shed feathers collected opportunistically between 1995 and 2006. Seven breeding pairs of Powerful Owls were identified, from which shed feathers were collected during 2003, 2004 and 2005. By comparing DNA profiles, one pair of Owls was found to have occupied the same breeding site for 10 years (1995-2005). The dispersal or movements of five offspring from this pair was also determined to be either of two scenarios: (1) the juvenile moves from the natal territory; however, isn't breeding; and (2) the juvenile is recovered as part of a breeding pair. Two pairs of Owls breeding in the urban fringe habitat were closely related, but no incidences of extra-pair fertilisation were detected among pairs in either habitat. This study provides new information about the breeding behaviour and dispersal of the Powerful Owl, and shows the potential of using genetic data sourced from shed feathers for studying cryptic, rare or elusive species.
The Norfolk Island Boobook Ninox novaeseelandiae undulata is confined to the small, isolated Norfolk Island group, an Australian territory. On morphological and biogeographical grounds, it is here classified as a large, distinctive subspecies of the New Zealand Morepork N. novaeseelandiae. In 1986 only one specimen, a female, survived. A shortage of large trees with suitable nesting holes appeared to be the immediate problem. The Australian Nature Conservation Agency, islanders and New Zealand wildlife authorities have cooperated in an attempt to re-establish an owl population in situ. Nest-boxes were erected in trees in the area frequented by the female and were used readily as roosts. In September 1987, two male New Zealand Moreporks were introduced. The female paired with one male and produced four hybrid F offspring (in 1989 and 1990). Two of these paired in mid-1991 and have since produced five F offspring (two in 1993 and three in 1994). The original female remains paired but now appears to be reproductively senile. At present there seems to be a shortage of mature males, since two female offspring are paired and both lay eggs and attempt to incubate them in the same nest; and a lone female has established a territory. In early 1995 all eleven owls appeared to be alive in the wild. The effort is low-cost, requires relatively little manpower, is carried out with minimal disturbance to the owls, and goes hand in hand with other conservation programmes.
This paper reports on the use of artificial nest-boxes in the territory of a breeding pair of Masked Owls Tyto novaehollandiae occupying a fragmented mosaic of urban bushland near Newcastle, central coastal New South Wales. Nest-boxes were trialled as a mitigation measure to offset the loss of several identified natural roost-trees cleared during construction of a four-lane road and residential subdivision. The use of the nest-boxes by the Owls was monitored over a 5-year period. Nest-boxes proved useful in the short term as a substitute for natural tree-hollows by juvenile Masked Owls, during the transition from nest- to hollow-roosting. There was no evidence that the nest-boxes were used by the resident adult pair. Possible factors for this result are discussed, along with issues associated with applying nest-boxes as a management and research tool for large forest owls.
A pair of Powerful Owls Ninox strenua was studied at each of two sites near Melbourne, Victoria, for three years (1977-1979) and 15 years (1980-1994 inclusive) respectively, by diurnal and nocturnal observation. Home ranges were mapped, nest sites characterised and breeding chronology and success monitored. General observations at these and eight other sites, of roosting, courting, nesting, parental and juvenile behaviour, fledgling mortality, hunting, interspecific conflicts, bathing, and camouflage posing, are presented. The regularly used parts of the home ranges of two pairs were each estimated as c. 300 ha, although for one pair this applied only to the breeding season. One pair used seven nest trees in 15 years, commonly two or three times each (range 1-4 times) over consecutive years before changing trees. Nest-switching may have been encouraged by human inspection of hollows. Nest entrances were 8-40 m (mean 22 m) above ground. The owls clearly preferred the larger and older trees (estimated 350-500+ years old), beside permanent creeks rather than seasonal streams, and in gullies or on sheltered aspects rather than ridges. Laying dates were spread over a month from late May, with a peak in mid June. The breeding cycle occupied three months from laying to fledging, of which the nestling period lasted 8-9 weeks. Breeding success was 1.4 young per pair per year and 94% nest success; early nests in gullies were more successful than late nests on slopes. The post-fledging dependence period lasted 6-7 months from fledging. The diet consisted mostly of possums and gliders, primarily the Common Ringtail Possum Pseudocheirus peregrinus (78-89% by number). A resident breeding pair of owls took at least 95 major prey items in 368 days.
This paper describes significant new information on the breeding biology of the Powerful Owl Ninox strenua from Victoria and New South Wales. The observations were made in areas of eucalypt forest within the urban fringes of major cities (Melbourne and Sydney) and in more secluded forest near Eden in south-eastern NSW. A banded first year female Powerful Owl dispersed from her birthplace to mate with a recently widowed, neighbouring male at a nest site 900 metres from her birthplace. She layed eggs at twelve months of age, and successfully reared two young in the nest used by her predecessor during the previous year. This is the only documented case of a banded nestling Powerful Owl being found outside its natal area. Evidence is presented to suggest that males may be responsible for selecting nest sites. Two instances are reported of males raising chicks following the death of their mates. Recently widowed males at three sites were observed to pair opportunistically with new females, including one case where murder was suspected. On several occasions, non-breeding floaters were observed in the territories of established breeding pairs.
Since European settlement of Australia, the dry open forests and woodlands of central Victoria have been extensively cleared and most large trees harvested, resulting in a decline of arboreal mammal populations. The Powerful Owl, which was formerly reliant on these prey species, still persists in the region but at very low densities and uncertain viability. Previous research has shown that Powerful Owls select home-ranges with more large trees and hollows than the forest at large, but the amount of such habitat that is required remained undefined. Four adult Powerful Owls (two males and two females) from four pairs occupying geographically separate territories in box-ironbark forest were radio-tracked over 1-6 months. Home-range size was much greater than previously assumed for this species (minimum convex polygon of 4774, 2896, 1770 and 1382 ha). Range-length was 5.7-8.9 km, and on average 5-12% of each home-range was used during a single night. Core foraging areas comprised many, typically small, patches scattered across the entire home-range. Selection of roosting sites was flexible and did not constrain spatial use of home-range, with 96% of roosts in very small to medium-sized trees, which are widely distributed. The finding of unexpectedly large foraging ranges suggests that enhancement of habitat quality and mammalian prey abundance in currently occupied home-ranges is the foremost goal for forest managers if a viable population is to be sustained.