Thursday, January 10, 2019

Owls of Australia

I am just back from a trip to the island of Sulawesi and several small satellite islands, where I saw 13 owls species. On the way, I had a brief stop-over in Australia and managed to see 5 owl species with help from friends. So, I have 18 owl species to write about, starting with five from down under! 

The Sooty Owl complex is a little bit of an ornithological riddle. Some authorities consider it to be one species, others two species: “Greater” and “Lesser”, and still others have suggested that it may warrant treatment as three species with the arfaki subspecies of New Guinea probably being distinct. Still others have suggested that the taxon from New Guinea might be more closely related to the Lesser Sooty Owl of northern Queensland, despite being more similar in appearance to the Greater Sooty Owl. Whatever taxonomy you follow, Sooty Owls are certainly unique-looking – sooty grey all over with fine white spots! Sooty Owl(s) are in the Barn Owl family (Tytonidae), which reaches its peak diversity in Australasia (at least in our current era). The Sooty Owls are not easy to see since they inhabit rainforest for the most part so I was happy to get a glimpse, even if the resulting photo is poor quality:

Greater Sooty Owl (Tyto tenebricosa), Queensland, Australia, © Christian Artuso

The Australian Masked-Owl is another Barn Owl family (Tytonidae) member and, as its name suggests, it is endemic to Australia. Interestingly, this species comes in at least two morphs, from pale grey and white (like the youngster shown here) to a rich buff/chestnut. This is a forest-dwelling Tyto owl, typically associated with tall forests that rather open in structure, although a variety of habitat types are used (the individual shown here is in an introduced conifer species). These tall (mature) forests of course have trees which provide large enough cavities for nesting, a critical resource (on a continent where no woodpeckers occur to make them), although nesting in caves and on cliffs has also been recorded.  

Australian Masked-Owl (Tyto novaehollandiae), Queensland, Australia, © Christian Artuso
From the “barn owl” family, we switch to the typical owl family (Strigidae) and to a genus of typical owls with a mostly Australasian and Southern Hemisphere distribution (although a few reach Asia and the Northern Hemisphere). The owls of the genus Ninox are often called “boobooks” (an onomatopoeiac word describing the two-syllable call of the Southern Boobook, although many members of this genus have similar disyllabic calls). Ninox is the only genus of typical owls (Strigidae) that occur regularly in Australia. I found this pair of Southern Boobooks hanging around a cavity in a suburb of Sydney, as you can see in the second photo (one advantage of my habit of getting up at 4 am). The Southern Boobook is a common species found in much of Australia, southern New Guinea and on several islands in eastern Indonesia (Wallacea). There are at least 10 subspecies, considerable variation in colour and even call, and no taxonomic consensus on where species boundaries within the complex might occur. Like so many owls of the Southern Hemisphere, a lot of research is needed to clarify its status.   
Southern Boobook (Ninox novaeseelandiae), New South Wales, Australia, © Christian Artuso

I had only one day in Melbourne between flights and was fortunate to connect with PhD student Nick Bradsworth for an afternoon check on some of the Powerful Owls that he monitors as part of his PhD research This was an absolutely fantastic chance to learn about this spectacular species. I did however have a hard time choosing which photos to post of the eight Powerful Owls that we saw in just a couple of hours. Of the four Ninox species that make their home in mainland Australia, the Powerful Owl is the only endemic (if you treat Tasmanian Boobook as a separate species from the Morepork of New Zealand, which the IOC does not, then it too would be an Aussie endemic; plus there is of course the Christmas Islands Boobook that is endemic to Christmas Island, politically part of Australia but closer to Java). The Powerful Owl is also the largest species in the genus Ninox, reaching a massive 65 cm and 1.7 kg, and approximately the seventh largest extant owl species in the world (this depends on whether you consider weight or linear measurements). Unusual among owls in general, male Powerful Owls are larger than females, with a squarer head shape. Males are also larger than females in the Rufous Owl and the Barking Owl. Nick explained one hypothesis of why males are larger than females in the Powerful Owl, viz. that their larger size is important for capturing and lifting large and heavy prey items (that can be delivered to the female outside the nest or at the entrance), whereas smaller body size in females gives them greater access to tree cavities for nesting (since larger size would limit the cavities that can be used and thereby reduce availability of an important resource). Certainly, the Powerful Owl has to be one of the largest owl species that habitually uses “enclosed” tree cavities for nesting (although there are a few records of them nesting in broken-off stumps, as is common in species like the Great Grey Owl). The elongated shape of the Powerful Owl with a rather small head is also fascinating. There is a selection of males (larger, squarer head), females and white juveniles of various ages (darker with age) in the photos below:

Powerful Owl (Ninox strenua), Victoria, Australia, © Christian Artuso
The Barking Owl, so named for its gruff call, is quite a large Ninox species (though noticeably smaller than the Powerful Owl) that can reach 44 cm and 700 g. Like the Powerful Owl, it also has a rather elongated shape, small-looking head and a minimal (barely discernable) facial disk. It is resident in parts of Australia, New Guinea and the Moluccan Islands of Indonesia. The weak facial disk (and symmetrical ears) suggests that eye sight is used more than hearing in hunting. This is because the stiff feathers of an owl’s facial disk are very good at channelling sound to the ears. The Barking Owl is known to consume many insects, and to hunt them by grabbing them in flight by sallying out from a perch (they also catch other flying prey such as bats in this manner). The also take many beetles and other invertebrates on the ground. Not at all common in many parts of its Australian range, I was delighted to have this change to observe one roosting on a low branch and study its unique structure. I was also impressed by the crispness of the white spots on the wings. 
Barking Owl (Ninox connivens), Queensland, Australia, © Christian Artuso

Of course, this is not a complete set of Australian owls but I hope it serves as a useful introduction.        


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