Owlet-nightjar

The genus Aegotheles was introduced in 1827 by the naturalists Nicholas Vigors and Thomas Horsfield to accommodate a single species, Caprimulgus novaehollandiae Latham, 1790.^[2][3] This binomial name is considered to be a junior synonym of Caprimulgus cristatus, the Australian owlet-nightjar that was introduced by George Shaw earlier in 1790.^[4][5][6] The genus name means "nightjar" or "goatsucker" from Ancient Greek αιξ/aix, αιγος/aigos meaning "goat" and θηλαζω/thēlazō meaning "to suckle".^[7] The family Aegothelidae was introduced (as subfamily Aegothelinae within the family Caprimulgidae) in 1853 by the French naturalist Charles Lucien Bonaparte.^[8][9]

A comprehensive 2003 study analyzing mtDNA sequences of Cytochrome b and ATPase subunit 8 suggests that 12 living species of owlet-nightjar should be recognized, as well as another that became extinct early in the second millennium AD.^[10]

The relationship between the owlet-nightjars and the (traditional) Caprimulgiformes has long been controversial and obscure and remains so today: in the 19th century they were regarded as a subfamily of the frogmouths, and they are still generally considered to be related to the frogmouths and/or the nightjars. It appears though that they are not as closely related to either as previously thought, and that the owlet-nightjars share a more recent common ancestor with the Apodiformes.^[11] As has been suggested on occasion since morphological studies of the cranium in the 1960s,^[12] they are thus considered a distinct order, Aegotheliformes. This, the caprimulgiform lineage(s), and the Apodiformes, are postulated to form a clade called Cypselomorphae, with the owlet-nightjars and the Apodiformes forming the clade Daedalornithes.

In form and habits, however, they are very similar to both caprimulgiform group – or, at first glance, to small owls with huge eyes. The ancestors of the swifts and hummingbirds, two groups of birds which are morphologically very specialized, seem to have looked very similar to a small owlet-nightjar, possessing strong legs and a wide gape, while the legs and feet are very reduced in today's swifts and hummingbirds, and the bill is narrow in the latter.

Owlet-nightjars are an exclusively Australasian group, but close relatives apparently thrived all over Eurasia in the late Paleogene.

Molecular phylogenetic studies have shown the Aegotheliformes are sister to the Apodiformes containing the hummingbirds, swifts and treeswifts.^[13][14] The two orders shared a common ancestor around 57 million years ago.^[14]

The following cladogram is based on a 2003 molecular phylogenetic study that sampled three regions of mitochondrial DNA mainly extracted from museum specimens. Some of the nodes were not well supported by the data.^[10]

The IOC World Bird List (version 15.1) lists ten species of Aegotheliformes:^[15]

• Feline owlet-nightjar, Aegotheles insignis – montane New Guinea
• Starry owlet-nightjar, Aegotheles tatei – Fly/Elevala area of central east New Guinea and Amazon Bay area of far southeast New Guinea
• Moluccan owlet-nightjar, Aegotheles crinifrons – Halmahera and Bacan Islands (north Moluccas)
• Wallace's owlet-nightjar, Aegotheles wallacii – Waigeo (Raja Ampat Islands, northwest of New Guinea), Aru Islands (southwest of New Guinea), Bird's Head Peninsula (northwest New Guinea) and patchily in New Guinea
• Mountain owlet-nightjar, Aegotheles albertisi – montane New Guinea
• New Caledonian owlet-nightjar, Aegotheles savesi – Grande Terre (New Caledonia); likely extinct
• Barred owlet-nightjar, Aegotheles bennettii – central to southeast New Guinea
• Vogelkop owlet-nightjar, Aegotheles affinis –Arfak Mountains, Bird's Head Peninsula (northwest New Guinea)
• Karimui owlet-nightjar, Aegotheles terborghi – Karimui Basin (central south New Guinea)
• Australian owlet-nightjar, Aegotheles cristatus – central south, southeast New Guinea, Australia and Tasmania

A fossil proximal right tarsometatarsus (MNZ S42800) was found at the Bannockburn Formation of the Manuherikia Group near the Manuherikia River in Otago, New Zealand. Dating from the Early to Middle Miocene (Altonian, 19–16 million years ago), it seems to represent an owlet-nightjar ancestral to A. novaezealandiae.^[16] In 2022, an additional specimen from the same locality was described by Worthy et al. as a new extinct species of Aeotheles, A. zealandivetus. The holotype specimen is NMNZ S.52917, a distal right tarsometatarsus.^[17]