Owlet-nightjars
(Aegothelidae)
Class: Aves
Order: Caprimulgiformes
Suborder: Caprimulgi
Family: Aegothelidae
Thumbnail description
Small to medium-sized, cryptically colored nocturnal birds that perch upright; have large heads, large forward-facing eyes within a facial disk, rounded wings, small legs and feet, and long toes that bear long claws
Size
7–12 in (18–30 cm); 1.0–3.5 oz (29–98 g)
Number of genera, species
1 genus; 7 species
Habitat
Forest, woodlands, scrub, and savannas
Conservation status
Critically Endangered: 1 species
Distribution
Northern Moluccas, New Guinea and satellite islands, Australia, Tasmania, and New Caledonia
Evolution and systematics
Owlet-nightjars are the most distinctive group in the order Caprimulgiformes, showing characteristics that to some extent link this order with the owls (Strigiformes). Similarities between owlet-nightjars and owls include the facial disk with eyes directed forwards, hole nesting, unmarked white eggs, and certain anatomical features. Furthermore, DNA-hybridization studies point to early evolutionary divergence of owlet-nightjars from the other Caprimulgiformes, perhaps not long after the early ancestors of the order diverged from ancestral owls. The earliest fossil owlet-nightjars are known from the Upper Eocene, suggesting that the family evolved in the early Tertiary.
As of 2001, all living species of owlet-nightjars are placed in the genus Aegotheles. There is some uncertainty about how many species should be recognized, particularly about whether spangled owlet-nightjars (Aegotheles tatei) should be regarded as a distinct species or as a subspecies of feline owletnightjars (A. insignis). Further studies of these poorly known birds may also suggest that certain populations of other species that are currently regarded as subspecies might be better elevated to species rank.
Physical characteristics
Like other members of the Caprimulgiformes, owletnightjars have a relatively large head, large eyes, and a wide gape. The small hooked bill has a wide base. Rictal bristles (the rictus is the gape of a bird's mouth) are well developed at the base of the bill and these along with other whiskers (facial bristles) are thought to function in directing prey into the mouth and in protecting the eyes from hard insect prey; a tactile function has also been suggested. Flank feathers are elongated. The wings are rather long but rounded and the tail is moderately long and wedge shaped. The legs are relatively short, but less so than in most other Caprimulgiformes, and the toes are long, slender, and bear claws. Aegotheles novaezealandiae, a species known only by fossils from the Quaternary of New Zealand, had much larger feet and legs than any living species and reduced wings, suggesting that it was more terrestrial than any living species. The rare New Caledonian owlet-nightjar (A. savesi) has legs and wings that approach the rather strong legs and reduced wings of the extinct New Zealand form, suggesting that it may also have come to rely more on walking and less on flying than other owletnightjars, but the habits of the living bird are almost unknown.
The plumage coloration of owlet-nightjars consists of the beautifully marked shades of brown, gray, buff, rufous, and blackish that characterize the Caprimulgiformes and promote concealment from predators. Patterns of several species are dominated by fine dark barring whereas others have bold blotches or streaks, often with prominent facial stripes. Sexes are similar.
Distribution
Owlet-nightjars reach their greatest diversity in New Guinea, which has seven of the nine species. Among these are Archbold's (A. archboldi), barred (A. bennettii), and Wallace's (A. wallacii) owlet-nightjars, as well as a small population of the Australian owlet-nightjar (A. cristatus), which is the only species in Australia and Tasmania. The other two species are restricted to the Northern Moluccas (Moluccan owlet nightjar, A. crinifrons) or New Caledonia (New Caledonian owlet-nightjar). All species are sedentary, with movements apparently limited to local dispersal.
Habitat
Owlet-nightjars are mainly forest birds, with different species in New Guinea occurring in lowland rainforests, mid-mountain forests, or in montane forests up to the scrubby growths at the tree line. Only the Australian owlet-nightjar occurs extensively in more open habitats in addition to open forests and woodlands. They extend into scrub-covered country such as mallee in the Australian interior and into coastal mangroves and feed in grasslands that adjoin habitats with trees.
Behavior
Owlet-nightjars occupy concealed roosts during the day, commonly in holes in trees. Some species roost in the concealment afforded by dense vines. Unless they are disturbed, all activity occurs in near-darkness between late dusk and the early predawn light. Flight is swift and direct but usually of short duration. Very little is known of the social organization and other aspects of behavior of most species. The broad outlines of behavior are clear only for the better-studied Australian owletnightjar. In that species, a pair of birds lives all year within a large territory. The pair roost in close proximity throughout the year, although roost sites are frequently changed.
Nocturnal calls provide the main evidence of the presence and identity of owlet nightjar species, and there can be little doubt that the voice is the main means by which these birds communicate with each other. Different species produce characteristic churring, snarling, or whistling sounds that evidently function in defense of territories. The full vocabulary is unknown for most species, but sounds given by some include hissing during threat displays from nest holes.
Feeding ecology and diet
The diet of most owlet-nightjars consists mainly of insects, supplemented by other small invertebrates such as spiders and millipedes. The mountain owlet-nightjar (A. albertisi) includes earthworms in its diet. The food of several species remains completely unknown. The main feeding technique consists of sallying from perches to capture prey in the bill, often from the ground, but also sometimes from tree trunks or by fluttering against foliage. Each prey item is swallowed whole.
Reproductive biology
For several species of owlet-nightjars, nests have not been discovered. All described nests have been in holes, mainly tree holes. Australian owlet-nightjars sometimes nest in cavities of banks or buildings. Clutches are of one to five, slightly glossy, unmarked white eggs that, for unknown reasons, have unusually thick shells. Young hatch with white down which is replaced by a second coat of gray down before the juvenal feathers grow. Details of breeding are known only for the Australian species, in which incubation takes 25–27 days and is carried out at least mainly by the female, eggs hatch over several days, the young are fed by both parents, and fledging occurs after 21–32 days. No nest cleaning occurs so that droppings accumulate in the nest cavity.
Conservation status
New Caledonian owlet-nightjars were known only from a single specimen collected in 1880, and they were feared extinct until a sighting was reported in 1998. It is now regarded as Critically Endangered. None of the other species is thought to be seriously threatened, although several of those in New Guinea remain poorly known and susceptible to deforestation.
Significance to humans
The owlet-nightjars are among the most inconspicuous and retiring of nocturnal forest birds; even local tribespeople in New Guinea know little about them.
Species accounts
Feline owlet-nightjarMountain owlet-nightjar
Barred owlet-nightjar
Australian owlet-nightjar
Resources
Books:Holyoak, D.T. "Owlet-nightjars." In Handbook of the Birds of the World. Vol. 5, Barn-owls to Hummingbirds, edited by Josep del Hoyo, Andrew Elliott, and Jordi Sargatal. Barcelona: Lynx Edicions, 1999.
Holyoak, D.T. Nightjars and Their Allies. Oxford: Oxford University Press, 2001.
Periodicals:Pratt, T.K. "Evidence for a Previously Unrecognized Species of Owlet-Nightjar." Auk 117 (2000): 1–11.
Burrell, H. "Owlet-Nightjar Nestlings." Emu 13 (1914): 216–217.
[Article by: David T. Holyoak, PhD]
