True owls or typical owls (family Strigidae) are one of the two generally accepted families of owls, the other being the barn owls (Tytonidae). The Sibley-Ahlquist taxonomy unites the Caprimulgiformes with the owl order; here, the typical owls are a subfamily Striginae. This is unsupported by more recent research (see Cypselomorphae for details), but the relationships of the owls in general are still unresolved. This large family comprises around 189 living species in 25 genera. The typical owls have a cosmopolitan distribution and are found on every continent except Antarctica.
While typical owls (hereafter referred to simply as owls) vary greatly in size, with the smallest species, the Elf Owl
, being a hundred times smaller than the largest, the Eurasian Eagle Owl
, owls generally share an extremely similar body plan. They tend to have large heads short tails, cryptic plumage
and round facial discs around the eyes. The family is generally arboreal (with a few exceptions like the Burrowing Owl
) and obtain their food on the wing. The wings are large, broad, rounded and long. Like other birds of prey
many owl species exhibit reverse sexual dimorphism
in size, where females are larger than males (as opposed to the more typical situation in birds where males are larger).
Because of their nocturnal habits they tend not to exhibit sexual dimorphism in their plumage. The feathers are soft and the base of each is downy, allowing for silent flight. The toes and tarsus are feathered in some species, and more so in species at higher latitudes. Numerous species of owl in the genus Glaucidium and the Northern Hawk Owl have eye patches on the backs of their heads, apparently to convince other birds they are being watched at all times. Numerous nocturnal species have ear-tufts, feathers on the sides of the head that are thought to have a camouflage function, breaking up the outline of a roosting bird. The feathers of the facial disc are arranged in order to increase sound delivered to the ears. Hearing in owls is highly sensitive and the ears are asymmetrical allowing the owl to localise a sound. In addition to hearing owls have massive eyes relative to their body size. Contrary to popular belief, however, owls cannot see well in extreme dark and are able to see fine in the day.
Owls are generally nocturnal and spend much of the day roosting. They are often perceived as tame since they will allow people to approach quite closely before taking flight, but they are instead attempting to avoid detection. The cryptic plumage and inconspicuous locations adopted are an effort to avoid predators and mobbing
by small birds.
The nearly 200 extant species
are assigned to a number of genera
, which are in taxonomic order:
- Genus Megascops: screech-owls, some 20 species
- Genus Otus: scops-owls; probably paraphyletic, about 45 species
- Genus Pyrroglaux - Palau Owl
- Genus Gymnoglaux - Bare-legged Owl or Cuban Screech-owl
- Genus Ptilopsis - white-faced owls, 2 species
- Genus Mimizuku - Giant Scops-owl or Mindanao Eagle-owl
- Genus Bubo - horned owls, eagle-owls and fish-owls; paraphyletic with Nyctea, Ketupa and Scotopelia, some 25 species
- Genus Strix - earless owls, some 15 species
- Genus Ciccaba - 4 species
- Genus Lophostrix - Crested Owl
- Genus Jubula - Maned Owl
- Genus Pulsatrix - spectacled owls, 3 species
- Genus Surnia - Northern Hawk-owl
- Genus Glaucidium - pygmy owls, about 30-35 species
- Genus Xenoglaux - Long-whiskered Owlet
- Genus Micrathene - Elf Owl
- Genus Athene - 2-4 species (depending on whether Speotyto and Heteroglaux are included or not)
- Genus Aegolius - saw-whet owls, 4 species
- Genus Ninox - Australasian hawk-owls, some 20 species
- Genus Uroglaux - Papuan Hawk-owl
- Genus Pseudoscops - Jamaican Owl and possibly Striped Owl
- Genus Asio - eared owls, 6-7 species
- Genus Nesasio - Fearful Owl
- Mioglaux (Late Oligocene? - Early Miocene of WC Europe) - includes "Bubo" poirreiri
- Intutula (Early/Middle Miocene of WC Europe) - includes "Strix/Ninox" brevis
- Alasio (Middle Miocene of Vieux-Collonges, France) - includes "Strix" collongensis
- "Otus/Strix" wintershofensis - fossil (Early/Middle Miocene of Wintershof West, Germany) - may be close to extant genus Ninox (Olson 1985:131)
- "Strix" edwardsi - fossil (Middle Miocene of Grive-Saint-Alban, France)
- "Asio" pygmaeus - fossil (Early Pliocene of Odessa, Ukraine)
- Strigidae gen. et sp. indet. UMMP V31030 (Rexroad Late Pliocene of Kansas, USA) - Strix/Bubo? (Feduccia 1970)
- Ibiza Owl, Strigidae gen. et sp. indet. - prehistoric (Late Pleistocene/Holocene of Es Pouàs, Ibiza) - see Sánchez Marco 2004
The supposed fossil heron "Ardea" lignitum (Late Pliocene of Germany) was apparently a strigid owl, possibly close to Bubo (Olson 1985:167). The Early - Middle Eocene genus Palaeoglaux from west-central Europe is sometimes placed here, but given its age it is probably better considered an own family for the time being.
- (1970): Some birds of prey from the Upper Pliocene of Kansas. Auk 87(4): 795-797. PDF fulltext
- (1985): The fossil record of birds. In: Farner, D.S.; King, J.R. & Parkes, Kenneth C. (eds.): Avian Biology 8: 79-238. Academic Press, New York.
- (2004): Avian zoogeographical patterns during the Quaternary in the Mediterranean region and paleoclimatic interpretation. Ardeola 51(1): 91-132. PDF fulltext