True owl

True owl Temporal range:Early Eoceneto present
Eastern screech owl
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Aves
Order:	Strigiformes
Family:	Strigidae Leach,1819
Type genus
Strix Linnaeus,1758
Genera
some 25, see text
Synonyms
StriginaesensuSibley & Ahlquist

Thetrue owlsortypical owls(familyStrigidae) are one of the two generally accepted families ofowls,the other being thebarn owls(Tytonidae). This large family comprises 230 living or recently extinctspeciesin 24genera.The typical owls have acosmopolitan distributionand are found on every continent exceptAntarctica.

Morphology

Cross sectionedgreat grey owlspecimen showing the extent of the body plumage, Zoological Museum, Copenhagen

Skeleton of a true owl (Strigidae)

While typical owls (hereafter referred to simply as owls) vary greatly in size, with the smallest species, theelf owl,being a hundredth the size of the largest, theEurasian eagle-owlandBlakiston's fish owl,owls generally share an extremely similar body plan.They tend to have large heads, short tails, crypticplumage,and round facial discs around the eyes. The family is generallyarboreal(with a few exceptions like theburrowing owl) and obtain their food on the wing. The wings are large, broad, rounded, and long. As is the case with mostbirds of prey,in many owl speciesfemales are largerthan males.

Because of theirnocturnalhabits, they tend not to exhibitsexual dimorphismin their plumage. Specialized feathers and wing shape suppress the noise produced by flying, both taking off, flapping and gliding.This silent flight allows owls to hunt without being heard by their prey. Owls possess three physical attributes that are thought to contribute to their silent flight capability. First, on the leading edge of the wing, there is a comb of stiff feathers. Second, the trailing edge of the wing contains a flexible fringe.Finally, owls have downy material distributed on the tops of their wings that creates a compliant but rough surface (similar to that of a soft carpet). All these factors result in significant aerodynamic noise reductions.The toes and tarsi are feathered in some species, and more so in species at higher latitudes.Numerous species of owls in the genusGlaucidiumand thenorthern hawk-owlhave 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 acamouflagefunction, breaking up the outline of a roosting bird. The feathers of thefacial discare 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 multiple directions. Owls can pinpoint the position of prey, such as a squeaking mouse, by computing when the sound from the object reaches the owl's ears. If the sound reaches the left ear first, the mouse must be to the left of the owl. The owl's brain will then direct the head to directly face the mouse.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 well in the day.

Owls are also able to rotate their heads by as much as 270 degrees in either direction without damaging the blood vessels in their necks and heads, and without disrupting blood flow to their brains. Researchers have found four major biological adaptations that allow for this unique capability. First, in the neck there is a major artery, called the vertebral artery, that feeds the brain. This artery passes through bony holes in the vertebra. These bony holes are ten times larger in diameter than the artery that passes through them (extra space in the transverse foramina) which creates air pockets that allow for more movement of the artery when twisted. 12 of the 14 cervical vertebrae in the owl's neck have this adaptation. This vertebral artery also enters the neck higher up than it does in other birds. Instead of going in at the 14th cervical vertebrae, it enters in at the 12th cervical vertebrae. Finally, the small vessel connection between the carotid and the vertebral arteries allow the exchanging of blood between two blood vessels. These cross connections allow for uninterrupted blood flow to the brain. This means that even if one route is blocked during extreme head rotations, another route can continue blood circulation to the brain.

Several owl species also have fluorescent pigments calledporphyrinsunder their wings. A large group of pigments defined by nitrogen-containing pyrole rings, including chlorophyll and heme (in animal blood), make up the porphyrins. Other bird species will use porphyrins to pigment eggshells in the oviduct. Owl species, however, use porphyrins as a pigment in their plumage. Porphyrins are most prevalent in new feathers and are easily destroyed by sunlight. Porphyrin pigments in feathers fluoresce under UV light, allowing biologists to more accurately classify the age of owls. The relative ages of the feathers are differentiated by the intensity of fluorescence that they emit when theprimariesandsecondariesare exposed toblack light.This method helps to detect the subtle differences between third and fourth generation feathers, whereas looking at wear and color makes age determination difficult.

Niche competition

It has been noted that there is some competition forniche spacebetween thespotted owland thebarred owl(both of which are true owls). This competition is related todeforestation,and therefore a reduction in niche quantity and quality. This deforestation is more specifically the result ofoverloggingandforest fires.These two species of owl are known to traditionally live inmature forestsof old and tall trees, which at this point in time are mostly limited topublic lands.As niche overlap is occurring in these two families, there is a concern with the barred owls encroaching on the spotted owl'sNorth Americanhabitats, causing a decline of the spotted owl.As noted above, these species prefer mature forests which, due to deforestation, are at limited supply and take a long time to reestablish after deforestation has occurred. Because thenorthern spotted owlshares its territories and competes with other species, it is declining at a more rapid pace. This invasion by barred owls occurred about 50 years ago in thePacific Northwest,and despite their low numbers, they are considered aninvasive speciesbecause of the harm done to native spotted owls. In thiscompetition for resources,hunting locations and general niches, the barred owl is pushing the spotted owl tolocal extinction.It is thought that the rapid decrease in population size of spotted owls will cause atrophic cascade,since the spotted owls help provide a healthyecosystem.

Behaviour

Owls are generallynocturnaland/orcrepuscularand spend much of the dayroosting.They are often misperceived as ‘tame’ since they allow humans to approach quite closely before taking flight, but in reality they are attempting to avoid detection through stillness. Their crypticplumageand the inconspicuous locations they adopt are an effort to avoid predators andmobbingby small birds.

Communication

Owls, such as theeagle-owl,will use visual signaling inintraspecific communication(communication within the species), both in territorial habits and parent-offspring interactions. Some researchers believe owls can employ various visual signals in other situations involvingintraspecific interaction.Experimental evidence suggests thatowl fecesand the remains of prey can act as visual signals. This new type of signaling behavior could potentially indicate the owls' current reproductive state to intruders, including otherterritorialowls or non-breeding floaters. Feces are an ideal material for marking due to its minimal energetic costs, and can also continue to indicate territorial boundaries even when occupied in activities other than territorial defense. Preliminary evidence also suggests that owls will use feces and the feathers of their prey to signal their breeding status to members within the same species.

Migration

Some species of owl are migratory. One such species, thenorthern saw-whet owl,migrates south even when food and resources are ample in the north.

Habitat, climate and seasonal changes

Some owls have a higher survival rate and are more likely to reproduce in a habitat that contains a mixture of old growth forests and other vegetation types. Old growth forests provide ample dark areas for owls to hide from predatorsLike many organisms, spotted owls rely on forest fires to create their habitat and provide areas for foraging. Unfortunately, climate change and intentional fire suppression have altered natural fire habits. Owls avoid badly burned areas but they benefit from the mosaics of heterogeneous habitats created by fires. This is not to say that all fires are good for owls. Owls only thrive when fires are not of high severity and not large stand-replacing (high-severity fires that burn most of the vegetation) which create large canopy gaps that are not adequate for owls.

Parasites

Avian malaria orPlasmodium relictumaffects owls and specifically, 44% of northern and Californian spotted owls harbor 17 strains of the parasite. As mentioned in the niche competition section above, spotted owls and barred owls are in competition so their niche overlap may be resulting in the plasmodium parasite having more hosts in a concentrated area but this is not certain.

Predators

The main predators of owls are other species of owls. An example of this occurs with the northern saw-whet owl that lives in the northern U.S. and lives low to the ground in brushy areas typically of cedar forests. These owls eat mice, and perch in trees at eye level. Their main predators are barred owls and great horned owls.

Systematics

Spectacled owl(Pulsatrix perspicillata)

ThefamilyStrigidae was introduced by the English zoologistWilliam Elford Leachin a guide to the contents of theBritish Museumpublished in 1819.

Amolecular phylogeneticstudy of the owls by Jessie Salter and collaborators published in 2020 found that the family Strigidae was divided into twosisterclades and some of the traditional genera wereparaphyletic.The placement of three monotypic genera remained uncertain due to the degraded nature of the available DNA.Based on these resultsFrank Gill,Pamela Rasmussenand David Donsker updated the online list of world birds that they maintain on behalf of theInternational Ornithological Committee(IOC).

Thecladogrambelow is based on the results of the study by Salter and collaborators published in 2020.The subfamilies are those defined byEdward DickinsonandJames Van Remsen Jr.in 2013.A genetic study published in 2021 suggested that the genusScotopeliamay be embedded withinKetupa.

Blakiston's fish owl(Ketupa blakistoni) the largest species of owl.

Tawny owl(Strix aluco)

Strigidae

Ieraglaucinae Ninox– 36 species: hawk-owls and boobooks Uroglaux– Papuan hawk-owl (position uncertain) Surniinae Aegolius– 5 species Athene– 9 species Glaucidium– 29 species: pygmy owls Surnia– northern hawk-owl Margarobyas– bare-legged owl (position uncertain) Taenioptynx– 2 species Micrathene– elf owl Xenoglaux– long-whiskered owlet Striginae Otus– 58 species: scops owls Asio– 9 species in total Ptilopsis– 2 species Jubula– maned owl (position uncertain) Bubo– 10 species: eagle-owls, horned-owls and snowy owl Ketupa– 12 species: fish owls and eagle-owls Scotopelia– 3 species: fishing owls Megascops– 25 species: screech owls Gymnasio– Puerto Rican owl Psiloscops– flammulated owl Strix– 22 species: earless owls Lophostrix– crested owl Pulsatrix– 3 species

The 235 extant or recently extinctspeciesare assigned to 23genera:

Theforest owlet(Athene blewiti), one of thecritically endangeredowls found in the Central Indian Forest

• GenusUroglaux– Papuan hawk-owl
• GenusNinox– Australasian hawk-owls, 37 species of which one is recently extinct
• GenusMargarobyas– bare-legged owl or Cuban screech-owl
• GenusTaenioptynx– two species previous placed inGlaucidium
• GenusMicrathene– elf owl
• GenusXenoglaux– long-whiskered owlet
• GenusAegolius– saw-whet owls, five species of which one is recently extinct
• GenusAthene– nine species
• GenusSurnia– northern hawk-owl
• GenusGlaucidium– pygmy owls, 29 species
• GenusOtus– scops owls, 58 species including three extinct species formerly placed inMascarenotus
• GenusPtilopsis– white-faced owls, two species
• GenusAsio– eared owls, nine species
• GenusJubula– maned owl
• GenusBubo– eagle-owls, horned-owls and snowy owl, 10 species
• GenusScotopelia– fishing owls, three species
• GenusKetupa– fish owls and eagle-owls, 12 species (including 9 species previously placed inBubo)
• GenusPsiloscops– flammulated owl
• GenusGymnasio– Puerto Rican owl
• GenusMegascops– screech-owls, 25 species
• GenusPulsatrix– spectacled owls, three species
• GenusLophostrix– crested owl
• GenusStrix– earless owls, 22 species, including four previously placed inCiccaba

Crested owl(Lophostrix cristata)

Late Quaternary prehistoric extinctions

• GenusGrallistrix– stilt-owls, four species
  • Kaua‘i stilt-owl,Grallistrix auceps
  • Maui stilt-owl,Grallistrix erdmani
  • Moloka‘i stilt-owl,Grallistrix geleches
  • O‘ahu stilt-owl,Grallistrix orion
• GenusOrnimegalonyx– Caribbean giant owls, one or two species
  • Cuban giant owl,Ornimegalonyx oteroi
  • Ornimegalonyxsp. – probably subspecies ofO. oteroi
• GenusAsphaltoglaux
  • Asphalt miniature owl,Asphaltoglaux cecileae
• GenusOraristrix
  • La Brea owl,Oraristrix brea

The extinctCuban giant owl(†Ornimegalonyx oteroi)

Fossilrecord

• Mioglaux(Late Oligocene? – Early Miocene of WC Europe) – includes"Bubo" poirreiri
• Intulula(Early/Middle Miocene of WC Europe) – includes"Strix/Ninox" brevis
• Yarquen(Middle Miocene of Argentina)
• Alasio(Middle Miocene of Vieux-Collonges, France) – includes"Strix" collongensis

The fossil database for Strigiformes is highly diverse and shows an origin from ~60MYA into the Pleistocene. The maximum age range for the Strigiformes clade extends to 68.6MYA.

Placement unresolved:

• "Otus/Strix" wintershofensis–fossil(Early/Middle Miocene of Wintershof West, Germany) – may be close to extant genusNinox
• "Strix" edwardsi–fossil(Middle Miocene of Grive-Saint-Alban, France)
• "Asio" pygmaeus–fossil(Early Pliocene ofOdesa,Ukraine)
• Strigidae gen. et sp. indet. UMMP V31030 (Rexroad Late Pliocene of Kansas, USA) –Strix/Bubo?
• Ibiza owl,Strigidae gen. et sp. indet. –prehistoric(Late Pleistocene/Holocene of Es Pouàs, Ibiza)

The supposed fossilheron"Ardea" lignitum(Late Pliocene of Germany) was apparently a strigid owl, possibly close toBubo.The Early–MiddleEocenegenusPalaeoglauxfrom west-central Europe is sometimes placed here, but given its age, it is probably better considered its own family for the time being.

Bibliography

• Olson, Storrs L. (1985). The fossil record of birds.In:Farner, D.S.; King, J.R. & Parkes, Kenneth C. (eds.):Avian Biology8:79–238. Academic Press, New York.

External links

• ITIS – Strigidae Taxonomy
• Typical owl videoson the Internet Bird Collection
• The Owl Pagesabout owls – photos, calls, books, art, mythology and more.