Australian boobook


The Australian boobook is a species of owl native to mainland Australia, southern New Guinea, the island of Timor, and the Sunda Islands. Described by John Latham in 1801, it was generally considered to be the same species as the morepork of New Zealand until 1999. Its name is derived from its two-tone boo-book call. Eight subspecies of the Australian boobook are recognized, with three further subspecies being reclassified as separate species in 2019 due to their distinctive calls and genetics.
The smallest owl on the Australian mainland, the Australian boobook is long, with predominantly dark-brown plumage with prominent pale spots. It has grey-green or yellow-green eyes. It is generally nocturnal, though is sometimes active at dawn and dusk, retiring to roost in secluded spots in the foliage of trees. The Australian boobook feeds on insects and small vertebrates, hunting by pouncing on them from tree perches. Breeding takes place from late winter to early summer, using tree hollows as nesting sites. The International Union for Conservation of Nature has assessed the Australian boobook as being of least concern on account of its large range and apparently stable population.

Taxonomy

English ornithologist John Latham described the boobook owl as Strix boobook in 1801, writing about it in English, before giving it its scientific name, taking its species epithet from a local Dharug word for the bird. The species description was based on a painting by Thomas Watling of a bird—the holotype—in the Sydney district in the 1790s. John Gould described Athene marmorata in 1846 from a specimen in South Australia; this is regarded as a synonym. German naturalist Johann Jakob Kaup classified the two taxa into subgenus Spiloglaux of a new genus Ieraglaux in 1852, renaming S. boobook to Ieraglaux bubuk. In his 1865 Handbook to the Birds of Australia, Gould recognised three species, all of which he placed in the genus Spiloglaux: S. marmoratus from South Australia, S. boobook, which is widespread across the Australian mainland and Tasmania, and S. maculatus from southeastern Australia and Tasmania. Meanwhile, in India, English naturalist Brian Houghton Hodgson had established the genus Ninox in 1837, and his countryman Edward Blyth placed the Australian boobook in the new genus in 1849.
Australian boobook has been designated the official name by the International Ornithological Committee, changed from "southern boobook" in 2019 with the separation of some Indonesian subspecies. The common name comes from the two-tone call of the bird, and has also been transcribed as "mopoke". William Dawes recorded the name bōkbōk "an owl" in 1790 or 1791, in his transcription of the Dharug language, and English explorer George Caley had recorded the native name as buck-buck during the earliest days of the colony, reporting that early settlers had called it cuckoo owl as its call was reminiscent of the common cuckoo. He added, "The settlers in New South Wales are led away by the idea that everything is the reverse in that country to what it is in England; and the Cuckoo, as they call this bird, singing by night, is one of the instances they point out." Gould recorded local aboriginal names: Goor-goor-da, Mel-in-de-ye, and Koor-koo. Alternative common names include spotted owl and brown owl. The Ngarluma people of the western Pilbara knew it as gurrgumarlu. In the Yuwaaliyaay dialect of the Gamilaraay language of southeastern Australia, the Australian boobook is guurrguurr.
Dutch naturalist Gerlof Mees and German evolutionary biologist Ernst Mayr regarded the taxonomy of the boobook owl complex as extremely challenging, the latter remarking in 1943 that it was "one of the most difficult problems I have ever encountered". In his 1964 review of Australian owls, Mees treated Australian and New Zealand boobooks, along with several taxa from Indonesia and Papua New Guinea, as one species—Ninox novaeseelandiae—with 16 subspecies. In his 1968 book Nightwatchmen of the Bush and Plain, Australian naturalist David Fleay observed that the boobooks from Tasmania more closely resembled those of New Zealand than those from mainland Australia, though he followed Mees in treating them as a single species. The Australian boobook was split from the Tasmanian boobook and morepork in volume 5 of the in 1999, though several authors, including Australian ornithologists Les Christidis and Walter Boles, continued to treat the three taxa as a single species.
Examining both morphological and genetic characters in 2008, German biologist Michael Wink and colleagues concluded that the Australian boobook is distinct from the morepork and Tasmanian boobook, and that it is instead the sister taxon to the barking owl. A 2017 study by Singapore-based biologist Chyi Yin Gwee and colleagues analysing both multi-locus DNA and boobook calls confirmed a sister relationship of N. n. novaeseelandiae and N. leucopsis and their close relationship to N. connivens. Genetic and call analysis show the Christmas boobook to be very close to the Australian populations of the Australian boobook, leading Gwee and colleagues to suggest it be reclassified within this species.
Gwee and colleagues found that boobook populations on larger mountainous islands were more distinct from Australian stock, while those on flatter smaller islands were much more similar, suggesting that these locations were colonised much more recently after previous populations had become extinct.

Subspecies

Eight subspecies of Ninox boobook are recognised in version 10.1 of the IOC World Bird List, published in January 2020:
Three former subspecies of Ninox boobook have been classified as distinct species since 2017: namely, Rote boobook, Timor boobook, and Alor boobook.

Description

The smallest owl on the Australian mainland, the Australian boobook is long. The nominate subspecies is the largest. It has short, rounded wings and a short tail, with a compact silhouette in flight. Australian boobooks on the Australian mainland follow Bergmann's rule, in that birds from cooler and more southerly parts of the range tend to be larger. Thus, birds from the Canberra region weigh around while those from the Cape York Peninsula and Broome are around. Females tend to be a little larger and heavier than males, with males weighing and females.
The Australian boobook has generally dark brown head and upperparts, with white markings on the scapulars and spots on the wings. Its head lacks tufts common in other owls, and has a paler facial disk, with a white supercilium and dark brown ear coverts and cheeks. The brown feathers of the upper forehead, above the supercilium, and sides of the neck have yellow-brown highlights. The feathers of the lores, chin and throat are white with black shafts. The feathers of the underparts are mostly brown with white spots and dark blue-grey bases. The upper tail is dark brown with lighter brown bars and a grey fringe at the end, while the undertail is a lighter grey-brown. The female tends to be more prominently streaked than the male overall, though this is inconsistent and wide variation is seen. The eyes have been described as grey-green, green-yellow, or even light hazel. The bill is black with a pale blue-grey base and cere. The feet are greyish to pinkish brown with dark grey to blackish claws. The underparts are pale, ranging from buff to cream, and are streaked with brown. The overall colour is variable and does not appear to correspond to subspecies or region. In northern and central Australia, Mayr found that the colour of the plumage appears to correlate with the rainfall or humidity, paler birds being found in three disjunct areas, each around away from the other two: the western Kimberley and Pilbara, Sedan on the Cloncurry River, and around Ooldea, with darker birds found on Cape York and Melville Island.
Young Australian boobooks are usually paler than adults, and do not attain adult plumage properly until their third or fourth year. Juveniles have whitish underparts and foreneck, a larger and more prominent pale eyebrow and larger whitish spots on their upperparts. The tips of their feathers are white and fluffy, remnants of the nestlings' down. These are worn away over time, persisting longest on the head. The feathers of the head, neck and underparts are fluffier overall. Immatures in their second and third year have plumage more like adults, though their crowns are paler and more heavily streaked.

Similar species

On mainland Australia, it could be confused with the barking owl or the brown hawk-owl, a rare vagrant to the northwest, although the Australian boobook is easily distinguished by its squat posture and distinctive pale border to its face mask. The Tasmanian boobook has been recorded from southern Victoria, with one record from New South Wales. It has darker and more reddish upperparts with brighter white dots, with more prominent white dots and a yellow-brown tinge to the underparts. It has pinkish grey feet and golden eyes.

Distribution and habitat

The Australian boobook is found across Australia, although it is scarce in more arid regions such as western New South Wales, southwestern and western Queensland, much of South Australia away from the coast and interior Western Australia and Northern Territory. In drier areas it is generally found along watercourses such as the Darling and Paroo Rivers, and Lake Eyre basin. It is found on numerous offshore islands such as Groote Eylandt, Melville Island, Mornington Island in the Gulf of Carpentaria and many islands off eastern Australia. It is found in southern New Guinea, Timor and surrounding islands in Indonesia. It is found in a wide range of habitats, from forest and open woodland to scrubland and semidesert areas. In Australia, it resides in mainly eucalypt forests. It has adapted to landscapes altered by human activity and is found in farmland and suburban areas as long as some scattered trees are present.

Behaviour

The Australian boobook is a mainly nocturnal species, though it may be active at dawn and dusk. It is heard much more commonly than seen, being particularly vocal in breeding season. The characteristic two-note boo-book call or hoot can be heard up to away, the second note generally lower in pitch than the first. Calling takes place from sunset through till dawn, generally with a peak in the two hours after dusk and just before dawn. It can continue for several hours. The male's hooting is higher pitched and of shorter duration, and is heard much more commonly than that of the female. He uses it as a contact call and to advertise his territory to females, as well as when bringing food to his mate or even before mating. Birds give a harsher version of the call when mobbing intruders. Both sexes, though mainly the female, give a single monosyllabic hoot as an alarm call or warning. Australian boobooks also make a repetitive croaking or grunting call while courting, mating, or greeting, or as a response to other boobooks hooting. Birds may switch from croaks to hoots seamlessly. Males generally croak at the beginning of the evening, and when arriving at the nest with food. A purring or braying call is used by both sexes as a contact call around the nest and when bringing food to nestlings, or by the female to beg for food from the male. The female makes a low trill during courtship and nesting. Growls, high-pitched yelps, and screeches can be made when attacking intruders.
The Australian boobook maintains and defends a territory in breeding season; whether this continues for the rest of the year is unclear. It retires to densely foliated spots in trees in the daytime, each individual often having several roosting sites. Caves or ledges are alternative roosting sites if no suitable trees are available. Although unobtrusive, it may give itself away by droppings or pellets on the ground beneath. The Australian boobook is mobbed by passerines if discovered. It may allow people to approach to within.
The maximum age recorded from banding has been 15 years 11 months, in a bird caught and later caught again on Black Mountain in the Australian Capital Territory.

Breeding

Across Australia, breeding takes place from July to February, though peaks in October, and is generally earlier in more northern areas. Eggs are laid from August to October in Queensland, from September to November in New South Wales and southern Western Australia, July to September in central and northern Western Australia, in October and November in Victoria, and in September to December in South Australia.
The Australian boobook nests in holes in trees between 1 and 20 m above the ground. The holes are generally vertical, and mostly in eucalypts, though other trees such as coast banksia have been recorded. Some sites are reused by the species for up to 20 years, especially if broods have been successfully raised in them before. Boobooks may also evict other birds such as galahs to use their hollows, and have used sites abandoned by babblers, crows, and ravens. The male does more of the site preparation, such as lining the base of the hollow with leaves.
Two or three oval, white eggs, laid two to three days apart, are most commonly laid in a clutch, though one to five may be seen. They average 41.6 mm long by 35.5 mm wide, and are finely pitted. The female alone incubates the eggs, during which time she is fed by the male. She does leave the nest at dusk for around half an hour, sometimes to bathe. Incubation takes 30–31 days, with the young often hatching at the same time. Occasionally, the time between first and last eggs hatching can be a few days.
Newly hatched chicks are covered with whitish down, and are blind and largely helpless. Their eyes begin to open on day 6 and are fully open by day 15. The juvenile feathers begin growing through the down from days 7 to 10, covering the baby owls by two weeks of age. Their mother broods them continuously for the first week, then only in the day until the third week. Her partner brings food to the nest, which she tears into pieces before feeding the nestlings. The young regurgitate pellets and defecate in the nest, which becomes quite smelly. They leave the nest 5–6 weeks after hatching, by which time they are fully feathered, with downy head and underparts and short tails. The tail reaches its adult length by 65 to 70 days. Young boobooks then live in their parents' territory for a further 2–4 months before dispersing, losing the remainder of their downy feathers by around 5 months of age.
Brushtail possums and introduced cats and rats raid the nests for nestlings and eggs, and raptors such as the brown goshawk, grey goshawk, Australian masked owl, and probably powerful owl seize young birds. Juvenile Australian boobooks are at greater risk after bushfires and have perished after being tangled up in dodder or bidgee-widgee.

Feeding

The Australian boobook generally preys on mice, insects, particularly nocturnal beetles and moths, and birds the size of a house sparrow. A higher proportion of its diet is invertebrates compared with other Australian owls. Fieldwork in the vicinity of Canberra found that vertebrates made up more of the diet in autumn and particularly in winter. Although more invertebrates were eaten than vertebrates, they made up only 2.8% of the biomass consumed. Mammals were the predominant prey species, especially the house mouse, and also black rat, bush rat, and Gould's wattled bat. Birds including common starling, house sparrow, red-browed finch, common myna, red-rumped parrot, and white-browed babbler, and invertebrates including grasshoppers, crickets, beetles, cockroaches, moths, wolf spiders, and huntsman spiders were also consumed. A study in Victoria found that larger animals were eaten, including Baillon's crake, common ringtail possum and feral rabbit.
Using a fence, branch, or telegraph pole as a perch or vantage point from which to hunt, the Australian boobook pounces on prey then retreats to a tree or elevated place to eat it. It often hunts in open areas near trees, and also where prey is likely to congregate, such as mice near haystacks or barns, or flying insects near street- or house-lights.

Conservation status

A widespread and generally common species, the Australian boobook is listed as being a species of least concern by the International Union for Conservation of Nature, on account of its huge range and stable population, with no evidence of any significant decline. Like most species of owl, the Australian boobook is protected by the Convention on International Trade in Endangered Species of Wild Fauna and Flora with its placement on the Appendix II list of vulnerable species, which makes the import, export, and trade of listed wild-caught animals illegal. A decline has occurred on the Swan Coastal Plain north of Perth. There, Australian boobook owls are dying after eating the rodents people have killed with second-generation anticoagulant rat poison. Older poisons such as warfarin or coumatetralyl were unlikely to affect owls.