Following the discovery of a Powerful Owl in the Corridor, we have instigated a Citizen Science project to monitor and hopefully find ways to improve conditions for this bird.
Natura Pacific video on Powerful Owls
The Story so far…
Powerful Owl discovered on the fringe of Woowookarung Regional Park
A Powerful Owl has been sighted and photographed adjacent to Woowookarung Regional Park.
An early-morning walk and the watchful eye of Peter spotted the owl on the morning of Friday January 24. The image of the unidentified owl was later confirmed to be the endangered Powerful Owl. The age and sex of the photographed owl could not be determined, so whether the sighting is indicative of Powerful Owl breeding within the Park, or a wandering individual passing through the area is not known.
This discovery marks the potential for the Park to support this rare species and speaks volumes for the quality of habitat herein – The Powerful Owl needs large tracts of old-growth eucalypt forest and nesting hollows – to provide a functioning ecosystem with an adequate prey base to sustain Australia’s largest owl.
The owl atop the food chain, with an impressive wingspan of 140 cm, this nocturnal predator feeds on possums, fruit bats, birds, and beetles (don’t worry, Koalas are not on the menu!) to help maintain the delicate balance of the ecosystem, ensuring prey species do not become overpopulated. As such, Powerful Owls are very ecologically important!
According to online citizen science databases there have been only 5 sighting reports in Ballarat – EVER! The most recent example was in April 2018 in Buninyong, whilst another in Canadian in 2012.
A conservation priority for this species in the corridor requires rejecting habitat destroying development proposals, and upgrading of the surrounding Parks and Reserves conservation and protection measures to improve the ecology for the Powerful Owl habitat
Indeed, the habitat importance of Woowookarung Regional Park has skyrocketed since the owl was sighted, as it adds more reasons for the Parks surrounding corridor to be conserved and protected.
If you see an owl in the Corridor, try to take a photo and seek assistance with identifying the species in question. Please report it to FoCC facebook or website or Birdlife Ballarat or on an online citizen science database such as:
Birdata: https://birdlife.org.au/projects/powerful-owl-project
eBird. https://ebird.org/australia/home
A Transcript featuring Powerful Owl authority Jasmine Zelený
Determining age: Case study – Woowookarung Owl photographed on January 24 2020
Jasmine: I would not be able to sex the bird from that image but it did look like a younger bird to me (as in one of 2019’s juveniles) as it still had a lot of white around the head (see Figure 1). I don’t think it was any sort of artefact from the artificial light source on the bird. By the time they are around 6-7 months old they have full adult plumage, and the only way you can really separate them from mature birds is to wait until dusk and hear if they are still trilling [begging]. Nothing has been done on the dispersal of young birds, Deakin Uni has done plenty of work in the past few years tracking adult birds, but what the young birds do when they are kicked out of the immediate breeding area is still unknown.
Just to give you an idea of how quickly they change, this is an example I use in my workshops (see Figure 3). First bird is 2 months old (2 days out of the hollow), second 4 months old, last is roughly 6 months old. The first photo taken of the young bird (Figure 1, outlined in red) looked to be around the stage of the last bird in my photos. They lose the rest of the white fairly quickly.
Determining age: Case study – Woowookarung Owl photographed on February 11 2020
Jasmine: Yes, it’s very hard to tell, but it does appear darker (see Figure 3). I wouldn’t rule out it being the same bird. There is a lot of stuff in the way in the photo so I don’t know if that is contributing to it looking so dark. You would have to listen to it vocalise to be sure, the young bird will definitely still be trilling at this age.
Determining sex
Jasmine: In regards to sexing the birds, it can often be difficult in the field, especially if you just have one bird. Males are larger, but again, size can be hard to gauge with only one bird (see Figure 4). Males also have a broader chest, and a square head with a flatter crown. It is important to note that Ninox owls in particular often manipulate the way their head looks in response to different stimuli. For example, they may puff up and make their head look really round if they feel threatened. The guys at Deakin have also mentioned to me that the females have a larger white patch above the bill. I haven’t really found this in any literature, but they have trapped and handled many Powerful Owls over the years, so I assume if they’re noticing this trend it’s probably just something that has gone unnoticed until now.
The edging of the facemask is a really useful feature (see Figure 5) that they [Deakin University researchers] have picked up on, with the mask going right to the edge of the head in female birds. The mask does not extend to the edge of the head in males due to their wider head (see Figure 6). We used the wide head to sex male birds before but nobody had picked up on the mask which makes it easier. Figure 2: Woowookarung owl photographed by Rob on the evening of Feb 25. This owl was reliably confirmed a male by Jasmine Zelený, using the facial mask sexing method. Notice how the face is wider than the facial mask.
Territorial aggression towards humans
Jasmine: We haven’t had any incidents with aggressive birds up here in South East Queensland, however there have been quite a few swooping incidents in NSW where birds have made contact with observers who I personally think were pushing their luck and not reading obvious signs of discomfort. You’re most likely to see aggressive behaviour from the male bird when chicks are still inside the hollow. If a Powerful Owl is uncomfortable with you there are a few things to look out for. If they are paying way too much attention to you and they’re not just going on with natural behaviours, then it’s probably a good idea to back off. If they ball up one of their feet and knock it on the perch, then that’s a serious warning to back off. If they let out one big deep hoot, run – haha! Once the chicks have actually fledged the adult birds tend to be fine. If you ever hear a demonic sheep sound come from a Powerful Owl, don’t be alarmed. It is called bleating and it is a very normal vocalisation. We have noticed they use it in a variety of situations. Sometimes they bleat to communicate with their chicks, we have had them bleating before mating, and we have even had a bird bleating in response to a Torresian Crow annoying it at a day roost. So, if you hear that bleating call, the Powerful Owl isn’t going to attack you.
When to conduct surveys
Jasmine: In terms of the breeding period, it’s quite variable as you move from the northern part of the range down south. Our birds are generally the earliest to breed and fledge young and Victorian birds are the latest. Here in South East Queensland we start to get our volunteers out in March/April [April/May for Victoria?] to do dusk surveys, this is when our Powerful Owls are reaffirming their bonds and getting ready to mate. It’s the time of year where they will be calling frequently. Right now, it’s kind of the off period for them.
Call playback ethics
Jasmine: Call playback (audio recordings of owl vocalisations) is used in moderation in order to detect whether owls are present in certain areas, however, it is completely useless when it comes to trying to locate a nest. If you use call playback, you’re going to be pulling the owl away from the breeding area. We also don’t use call playback in active territories where we know there are owls, there is just no point and it’s disruptive. Our current survey protocol for investigating new sites is as follows: calls will not be played more frequently than for 10 seconds during each minute of a ten-minute survey period, and call playback will not be used at any point location or within 500m of that location more than 5 times in a calendar year. I am pretty sure we are going to cut this back even more this year.
How to locate nest hollows
Jasmine: If you are wanting to find the hollow, it will take some serious work. It helps to first find their roost [favourite perch] site and follow them through the season. The nest tree normally isn’t too far from their regular roost. Powerful Owls favour creek lines and gullies for roosting due to the denser vegetation associated with them as well as providing slightly cooler temperatures compared to the surrounds. You will need to look for signs such as whitewash (faeces), [regurgitated] pellets, as well as traces such as bits and pieces of their food they may have not found to be too palatable (for example I often find flying fox wings snipped off). Once you find a roost it is easier to keep track of the birds. Once the female is in the nest hollow, the male will usually be roosting within sight of the nest tree to keep an eye on things and protect it from any potential predators. If you can find where he is roosting during this period, dusk watches can be very productive. The male will sometimes pop his head into the hollow before he leaves to hunt. If he is holding onto food at his day roost, there is a very high chance he will drop it into the hollow for the female and chicks just after dark before he goes hunting. I have found hollows this way. Another way we have found plenty of hollows is to do dusk walks during the time where we would expect chicks to have a louder trill and to be close to fledging. So up here in SEQ that can be anywhere from mid-July to September. I expect it may be a month or so behind in Victoria [mid-August to October], but there may be exceptions. Last year a whole bunch of hollows were found this way, not too many trees are going to start trilling at you – haha!
Acknowledgments: Friends of Canadian Corridor wishes to extend its most grateful thanks to Jasmine Zelený who generously shared her extensive knowledge, insights, and photography, and kindly consented to its publication.
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Take-home messages and Joel’s background research
- Woowookarung owl: The owl photographed by Peter (until a better alternative is raised, I propose we name he/she ‘Woowoo’; a play on words between its postal address and the sound of a typical Powerful Owl vocalisation), reliably believed to be a juvenile (as of January 24), would theoretically indicate this juvenile and its parents are resident in the Woowookarung bushland! As Jasmine mentions, little is known about the dispersal habits of young owls that have newly reached independence from their parents. However, Menkhorst & Loyn (2005) posited that autumn is when there tends to be passerby immature (adolescent) owls wandering through the area, seeking to establish a territory of their own. This would further strengthen the likelihood that Peter’s photographed owl is a Woowookarung resident.
- Approximating Woowoo’s age: It must be cautioned that there are some inconsistencies in the literature in relation to breeding timelines, whether born out of technicality, incomplete knowledge or variability between QLD and Victorian Powerful Owl populations. For example, the breeding period (this term in itself can be vague as it could encompass some, or the full suite, of courtship, mating, and laying, incubating and hatching of eggs, and rearing chicks) according to Pizzey & Knight (2013) is from June to September, which is contradicted by Menkhorst et al. (2019) who say they lay eggs from May to June (with eggs hatching within 36-38 days). Morecombe (2010) suggests April to September, which is a very broad timeframe but likely the one safest to assume inclusive of Victorian Powerful Owls that are, as Jasmine says, roughly a month behind the QLD ones. Whatever the case, Woowoo’s plumage (as of January 24) – at least partly juvenile – suggests he/she was younger 6-7 months on January 24. This 6-7 month milestone, as Jasmine says, is when they graduate to full adult plumage, but remain dependent on the parents for a little while longer, for the time being that they continue their trilling (begging) call. As for lifespan, in my review of the literature, the oldest age I came across was a breeding pair studied over the course of 15 years (McNabb 1996), while BirdLife International (2016) estimates a generation length of 4.1 years (bearing in mind that ‘generation length’ is defined as ‘the average age of parents of the current cohort, reflecting the turnover rate of breeding individuals in a population’).
- Habitat selection: Powerful Owls select habitat patches based largely on prey abundance and hollow availability. Hollow availability is critical for two reasons: providing safe haven for mother and young Powerful Owls, but also for supporting populations of prey species, e.g. Sugar Glider (seemingly the second-most preferred prey item for Victorian Powerful Owls, according to a number of references) and Common Brushtail Possum – both of which are hollow-occupiers. Some of its avian prey are also hollow-nesters.
- Nest hollow selection: The large size of Powerful Owl (60-66cm) would necessitate large, old gumtrees be of 150-500+ years old (Roudavski & Parker 2020) to ensure a suitably-sized hollow to accommodate mother and 1-2 young. This reaffirms the importance not only of old-growth trees in the Canadian Corridor, but also the forests aged 70-odd years old (I believe that was an age class within the Park, quoted by Jeff) as they need to be preserved long beyond our legacy to become successors of the current old-growth trees for when they fall over or are removed. Further, for means of locating potential nests, the hollows utilised have been found to range from heights of 8-40m (McNabb 1996).
- Artificial nest-boxes: A McNabb & Greenwood (2011) study confirmed Powerful Owls do not completely disregard nest boxes. However, Roudavski & Parker (2020) caution that the one documented case of artificial nest boxes supporting breeding success was dampened by the death of one of the two chicks. The link below leads to the journal article which documents artificial nest-box use by Powerful Owl and discusses nest-box issues and design (please bear in mind that this article requires a fee for access).
https://search.informit.com.au/documentSummary;dn=301402238496529;res=IELHSS - Territory size: Powerful Owl pairs occupy a home range (territory) estimated to be 300ha (Chafer 1996). Compare this to the spatial area of Woowookarung Regional Park (I believe Jeff said was roughly 700ha, but need to verify this). It must be cautioned that these are estimates, and again availability of hollows and prey abundance is a big factor.
- Diet: The diet of Powerful Owl tends to favour nocturnal arboreal (tree-dwelling) marsupials, e.g. the apparent favourite being Common Ringtail, however they are adaptable to whatever prey is in high abundance, including to such small sizes as Christmas Beetles. Other prey items pertinent to Ballarat include birds e.g. currawongs, magpies, cockatoos, lorikeets, rosellas, kookaburras, Tawny Frogmouths, and even goshawks (various literature), and introduced mammals (pest control services!) such as Black Rat, rabbits, and even feral cats (Chafer 1992) – lock up your cats, locals!
Chart to distinguish Powerful Owls from other local owls and frogmouth.
| Species | Parameter | |||||
| Size | Eyes / Face | Breast | Voice | Ballarat sightings* | Other | |
| Powerful Owl | 60-65 cm | Bold yellow eyes | Barred with dark greyish brown wide ‘V’ pattern | Classic owl noise of ‘whoo-hoo’ | 32+ | Massive yellow feet |
| Australian (Southern) Boobook | 25-36 cm Rosella-sized but stumpier |
Pale-rimmed ‘goggles’ | Splotchy | ‘Boo-book’ | ~35 | Habitat versatility, e.g. roosts in sheds |
| Barking Owl | 35-46 cm
Magpie-sized but stumpier |
Almost-comically large eyes | Brown vertical streaks | Dog-like barking | 5 | Upright perching posture |
| Eastern Barn Owl | 30-40 cm | Face ‘disc’ rather than ‘masks’ of above-listed | White | Screechy | 7 | Tyto that prefers open country, rather than forests as per above-listed Ninox spp. |
| Tawny Frogmouth | 33-50 cm | Wide mouth; Whiskers |
Grey with fine black streaks spaced far apart | Humming ‘oom oom’ | Innumerable | Urban-adapted Not an owl |
Note: * Ballarat sightings is an entire record of historical sightings reported to eBird, Birdata, and Atlas of Living Australia
References
BirdLife International (2016). Ninox strenua. The IUCN Red List of Threatened Species.
Chafer, C. (1992) ‘Observations of the Powerful Owl Ninox strenua in the Illawarra and Shoalhaven Regions
of New South Wales’.
Clemens, R. (2020) Powerful Owl Brisbane Project update. https://cm.birdlife.org.au/campaigns/reports/viewCampaign.aspx?d=r&c=E939C751A24DAF4F&ID=F276D9796B5F3DD22540EF23F30FEDED&temp=False&tx=0
Cooke, R., et al. (2020) ‘Photography can determine the sex of a predator with limited sexual dimorphism: A case study of the Powerful Owl’.
McNabb, E. (1996) ‘Observations on the Biology of the Powerful Owl Ninox strenua in Southern Victoria’.
McNabb, E., Greenwood, J. (2011) ‘A Powerful Owl disperses into town and uses an artificial nest-box’.
Menkhorst, P., et al. (2019) ‘The Australian Bird Guide’.
Menkhorst, P., Loyn, R. (2005) ‘Diet of a Powerful Owl roosting in the Royal Botanic Gardens, Central Melbourne’.
Morcombe, M. (2010) ‘Birds of Australia’ [Digital Edition].
Pizzey, G., Knight, F. (2007) ‘Birds of Australia’ [Digital Edition].
Roudavski S., Parker D. (2020), Modelling Workflows for More-than-Human Design: Prosthetic Habitats for the Powerful Owl (Ninox strenua), In: Gengnagel C., Baverel O., Burry J., Ramsgaard Thomsen M., Weinzierl S. (eds) Impact: Design With All Senses. DMSB 2019. Springer, Cham, doi: https://doi.org/10.1007/978-3-030-29829-6_43
Zelený, J. (pers. comm.). The University of Queensland graduate – wildlife scientist and authority on nocturnal birds.