The Australian owlet-nightjar (Aegotheles cristatus) is a nocturnalbird found in open woodland across Australia and in southern New Guinea. It is the smallest nocturnal bird found in Australia which makes it prone to predation.[2] It is colloquially known as the moth owl or fairy owl[3] and Jarlajirrpi by the Warlpiripeople. It is the most common nocturnal bird in Australia. Despite suffering from predation and competition by introduced species it is not considered threatened.[1]
The Australian owlet-nightjar was formally described as Caprimulgus cristatus in 1790 by the English naturalist George Shaw based on a specimen collected in New South Wales, Australia.[4] The species was described in the same year by John Latham as Caprimulgus novaehollandiae but Shaw's account is believed to be earlier and thus has precedence.[5][6] The Australian owlet-nightjar was previously placed in Caprimulgiformes but updated taxonomy now places it in Aegotheliformes.[7] The Australian owlet-nightjar is now one of ten species placed in the genus Aegotheles that was introduced in 1827 by the naturalists Nicholas Vigors and Thomas Horsfield.[8] It is the type species of the genus.[9] The specific epithet cristatus is Latin meaning "crested", "plumed" or "tufted".[10]
A. c. tasmanicusMathews, 1918 – Tasmania (southeast Australia)
Description
Australian owlet-nightjar mainly nests in tree hollows.
The Australian owlet-nightjar is a small to medium-sized owlet-nightjar with grey upperparts and a white, barred front and a distinct dark and pale patterning on the head. It has large dark eyes that are non-reflective when a light is shone upon them, unlike many other nocturnal birds.[11] It is 21-25 cm long.[12] In the north of Australia females can also have a rufous morph. Males may have a rufous morph as well but it is not as distinct as the female's. This rufous morph is only in the North of Australia and is not found in coastal SE Australia.[13] Juvenile birds are similar to adults with slightly more spotting, shorter face bristles, and blurred markings on the face and nape.[14] The plumage is overall paler in desert populations. Australian owlet-nightjars are adapted to live in open woodland, with more pointed wings and larger feet, unlike most of the rest of the family that live in dense forest (though some individuals of this species can and do live in such habitat in Queensland and New Guinea).
Physiology
Thermoregulation
In summer Australian owlet-nightjars have higher body temperatures, and lower body temperatures in winter.[15] To combat lower temperatures in winter, Australian owlet-nightjars increase feather insulation.[15] During summer, Australian owlet-nightjars build up fat deposits which they have fully used up by mid-winter[15]. Australian owlet-nightjars are nocturnal hunters, so in winter they have the advantage of longer periods of darkness for hunting despite struggling with lower abundance of arthropods to eat.[15] While resting during the day, Australian owlet-nightjars select roosts that are warmed by the sun to lower the energy required to keep them warm.[15]
In hot weather birds use evaporative cooling to cool their body temperature.[16] This relies on panting and gular-fluttering. Being a nocturnal species, Australian owlet-nightjars are reluctant to leave roost sites during the day, and cannot seek out water to rehydrate that lost from panting.[16] In order to conserve water and avoid leaving their roost Australian owlet-nightjars have more efficient thermoregulation to prevent excessive moisture loss and dehydration.[16] This makes them highly heat tolerant. Australian owlet-nightjars roost in hollows which protects them from full exposure to the sun and heat during the day, which is possibly why they are slightly less heat-tolerant than related species that roost out in the open on arid ground[16].
Torpor
Australian owlet-nightjars are well known for frequently entering torpor, and during winter in Australia this usually happens daily.[17] They usually enter torpor once a day around dawn for 3-4 hours, if they enter torpor twice in a day it is always a morning bout followed by a bout in the afternoon or night.[17] Australian owlet-nightjars prefer roosting in treehollows, but will occasionally roost in rock cavities, which are warmer and more temperature stable.[17] In one study, Australian owlet-nightjars that roosted in rocks entered torpor half as often as those roosting in trees, and if they entered a second torpor it was more likely to be in the afternoon, while those in tree crevices were more likely to enter a second torpor at night.[17] Australian owlet-nightjars tend to enter torpor when the atmospheric temperature is below 6.8°C but may even do so at temperatures as high as 14.5°C.[17]
Torpor can be used to protect against fluctuating temperatures or food shortages by reducing the body’s metabolism, heart rate, and temperature.[18] Australian owlet-nightjars enter torpor more during periods of low arthropod abundance[18]. Temperatures during the breeding season are warm and there is an abundance of arthropods so Australian owlet-nightjars will not enter torpor during this period.[19]
Distribution and Habitats
Australian-owlet Nightjars are widespread in Australia and are hollow-dependent species, meaning they use tree hollows for both roosting and nesting.[20] In Australia they can be found in suburban and urban-fringe environments.[20]Rivers and waterways are important to this species, a study in Australia found that they were more likely to be found in areas with waterways than those without.[20]
The Australian owlet-nightjar is frequently heard but not often seen. Adult Australian owlet-nightjars appear to have four main calls described as: “churr, cry, yuk, cho-ok, hiss, screech and other short harsh rattling notes”.[7]
Roosting
Australian owlet-nightjars prefer roosting in tree cavities that are closer to the ground and prefer trees with lots of other cavities that are close to trees with more cavities.[22] On average they move between roost sites over a distance of about 300m every 9 days and in one study used between 2-6 different cavities over a 6 month period.[22] The same study found that Australian owlet-nightjars tended to roost alone and favoured Stringybark and Manna Gum.[22] They were also found in Yellow Box, Red Gum, Apple Box, and two dead Eucalyptus trees that could not be identified to species.[22] Cavity choice is essential as Australian owlet-nightjars spend more than half their time roosting.[22] It is still unknown why Australian owlet-nightjars use hollows to roost as many related species roost in the open on tree limbs or on the ground.[22] Some hypotheses are predation and ectoparasite avoidance based on similar behaviour in bats but further research is needed.[22]
Normally Australian owlet-nightjars roost tree hollows and snags but there are records of them roosting in man-made structures like forestry boom gates in Tasmania.[23] These gates are used to restrict access to private and crown land and when opened occasionally flush Australian owlet-nightjars that were roosting inside. They have also been found roosting in roof cavities andchimneys.[23]
Hunting and diet
The Australian owlet-nightjar feeds at night by diving from perches and snatching insects from the air, ground or off trunks and branches, in the manner of a flycatcher. It may also feed on the wing. It feeds on most insects, particularly beetles, grasshoppers and ants. During the day they roost in hollows in trees, partly for protection from predators and partly to avoid being mobbed by other birds that mistake them for owls.[24]
The Australian owlet-nightjar nests mainly in holes in trees (or in other holes and crevices), which is provisioned with leaves by both of the pair. It is thought that the frequent addition of eucalyptus leaves is because they act as a beneficial insecticide. Three or four eggs are laid, and incubated by the female for just under a month. Both the adults feed the chicks, which fledge after a month. The young birds are reported to stay close to the parents for several months after they fledge.[24]
1 2 Holyoak, D.T. (1999). "Family Aegothelidae (Owlet-nightjars)". In del Hoyo, J.; Elliott, A.; Sargatal, J. (eds.). Handbook of the Birds of the World. Vol.5: Barn-owls to Hummingbirds. Barcelona, Spain: Lynx Edicions. pp.252–265 [264]. ISBN978-84-87334-25-2.
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