Owls are divided into twofamilies: the true (or typical) owl family,Strigidae, and thebarn owl andbay owl family,Tytonidae.[2] Owls hunt mostly smallmammals,insects, and other birds, although a few species specialize in huntingfish. They are found in all regions of the Earth except the polar ice caps and some remote islands.
Owls possess large, forward-facing eyes and ear-holes, ahawk-likebeak, a flat face, and usually a conspicuous circle of feathers, afacial disc, around each eye. The feathers making up this disc can be adjusted to sharply focus sounds from varying distances onto the owls' asymmetrically placed ear cavities. Most birds of prey have eyes on the sides of their heads, but the stereoscopic nature of the owl's forward-facing eyes permits the greater sense ofdepth perception necessary for low-light hunting. Owls havebinocular vision, but they must rotate their entire heads to change the focus of their view because, like most birds, their eyes are fixed in their sockets. Owls are farsighted and cannot clearly see anything nearer than a few centimetres of their eyes. Caught prey can be felt by owls with the use offiloplumes—hairlike feathers on the beak and feet that act as "feelers". Their far vision, particularly in low light, is exceptionally good.
Owls can rotate their heads and necks as much as 270°. Owls have 14 neck vertebrae — humans have only seven — and their vertebral circulatory systems are adapted to allow them to rotate their heads withoutcutting off blood to the brain. Specifically, theforamina in their vertebrae through which the vertebral arteries pass are about ten times the diameter of the artery, instead of about the same size as the artery, as is the case in humans; the vertebral arteries enter thecervical vertebrae higher than in other birds, giving the vessels some slack, and thecarotid arteries unite in a very largeanastomosis or junction, the largest of any bird's, preventing blood supply from being cut off while they rotate their necks. Other anastomoses between the carotid and vertebral arteries support this effect.[4][5]
The smallest owl—weighing as little as 31 g (1+3⁄32 oz) and measuring some13.5 cm (5+1⁄4 in)—is theelf owl (Micrathene whitneyi).[6] Around the same diminutive length, although slightly heavier, are the lesser knownlong-whiskered owlet (Xenoglaux loweryi) andTamaulipas pygmy owl (Glaucidium sanchezi).[6] The largest owls are two similarly sizedeagle owls; theEurasian eagle-owl (Bubo bubo) andBlakiston's fish owl (Bubo blakistoni). The largest females of these species are 71 cm (28 in) long, have a 190 cm (75 in) wing span, and weigh4.2 kg (9+1⁄4 lb).[6][7][8][9][10]
Different species of owls produce different sounds; this distribution of calls aids owls in finding mates or announcing their presence to potential competitors, and also aidsornithologists andbirders in locating these birds and distinguishing species. As noted above, their facial discs help owls to funnel the sound of prey to their ears. In many species, these discs are placed asymmetrically, for better directional location.
Owlplumage is generallycryptic, although several species have facial and head markings, including face masks,ear tufts, and brightly coloredirises. These markings are generally more common in species inhabiting open habitats, and are thought to be used in signaling with other owls in low-light conditions.[11]
Sexual dimorphism
A stamp from the Soviet Union, 1979
Sexual dimorphism is a physical difference between males and females of a species. Female owls are typically larger than the males.[12] The degree of size dimorphism varies across multiple populations and species, and is measured through various traits, such as wing span and body mass.[12]
One theory suggests that selection has led males to be smaller because it allows them to be efficientforagers. The ability to obtain more food is advantageous during breeding season. In some species, female owls stay at their nest with their eggs while it is the responsibility of the male to bring back food to the nest.[13] If food is scarce, the male first feeds himself before feeding the female.[14] Small birds, which are agile, are an important source of food for owls. Male burrowing owls have been observed to have longer wing chords than females, despite being smaller than females.[14] Furthermore, owls have been observed to be roughly the same size as their prey.[14] This has also been observed in other predatory birds,[13] which suggests that owls with smaller bodies and long wing chords have been selected for because of the increased agility and speed that allows them to catch their prey.[citation needed]
Another popular theory suggests that females have not been selected to be smaller like male owls because of their sexual roles. In many species, female owls may not leave the nest. Therefore, females may have a larger mass to allow them to go for a longer period of time without starving. For example, one hypothesized sexual role is that larger females are more capable of dismembering prey and feeding it to their young, hence female owls are larger than their male counterparts.[12]
A different theory suggests that the size difference between male and females is due to sexual selection: since large females can choose their mate and may violently reject a male's sexual advances, smaller male owls that have the ability to escape unreceptive females are more likely to have been selected.[14]
If the character is stable, there can be different optimums for both sexes. Selection operates on both sexes at the same time; therefore it is necessary to explain not only why one of the sexes is relatively bigger, but also why the other sex is smaller.[15] If owls are still evolving toward smaller bodies and longer wing chords, according to V. Geodakyan's Evolutionary Theory of Sex, males should be more advanced on these characters. Males are viewed as an evolutionary vanguard of a population, and sexual dimorphism on the character, as an evolutionary "distance" between the sexes. "Phylogenetic rule of sexual dimorphism" states that if there exists a sexual dimorphism on any character, then the evolution of this trait goes from the female form toward the male one.[16]
Hunting adaptations
All owls are carnivorous birds of prey and live on diets of insects, small rodents and lagomorphs. Some owls are also specifically adapted to hunt fish. They are very adept in hunting in their respective environments. Since owls can be found in nearly all parts of the world and across a multitude of ecosystems, their hunting skills and characteristics vary slightly from species to species, though most characteristics are shared among all species.[17]
Most owls share an innate ability to fly almost silently and also more slowly in comparison to other birds of prey. Most owls live a mainly nocturnal lifestyle and being able to fly without making any noise gives them a strong advantage over prey alert to the slightest sound in the night. A silent, slow flight is not as necessary for diurnal and crepuscular owls given that prey can usually see an owl approaching. Owls' feathers are generally larger than the average birds' feathers, have fewer radiates, longer pennulum, and achieve smooth edges with differentrachis structures.[18] Serrated edges along the owl's remiges bring the flapping of the wing down to a nearly silent mechanism. The serrations are more likely reducing aerodynamic disturbances, rather than simply reducing noise.[19] The surface of the flight feathers is covered with a velvety structure that absorbs the sound of the wing moving. These unique structures reduce noise frequencies above 2 kHz,[20] making the sound level emitted drop below the typical hearing spectrum of the owl's usual prey[20][21] and also within the owl's own best hearing range.[22][23] This optimizes the owl's ability to silently fly to capture prey without the prey hearing the owl first as it flies, and to hear any noise the prey makes. It also allows the owl to monitor the sound output from its flight pattern.
The disadvantage of such feather adaptations for barn owls is that their feathers are not waterproof.[24] The adaptations mean that barn owls do not use theuropygial gland, informally the "preen" or "oil" gland, as most birds do, to spread oils across their plumage through preening.[25] This makes them highly vulnerable to heavy rain when they are unable to hunt.[26] Historically, they would switch to hunting indoors in wet weather, usingbarns and other agricultural buildings, but the decline in the numbers of these structures in the 20th and 21st centuries has reduced such opportunities.[24] The lack of waterproofing means that barn owls are also susceptible to drowning, indrinking troughs and other structures with smooth sides. TheBarn Owl Trust provides advice on how this can be mitigated, by the installation of floats.[27]
Vision
Eyesight is a particular characteristic of the owl that aids in nocturnal prey capture. Owls are part of a small group of birds that live nocturnally, but do not useecholocation to guide them in flight in low-light situations. Owls are known for their disproportionally large eyes in comparison to their skulls. An apparent consequence of the evolution of an absolutely large eye in a relatively small skull is that the eye of the owl has become tubular in shape. This shape is found in other so-called nocturnal eyes, such as the eyes ofstrepsirrhineprimates andbathypelagic fishes.[28] Since the eyes are fixed into these sclerotic tubes, they are unable to move the eyes in any direction.[29] Instead of moving their eyes, owls swivel their heads to view their surroundings. Owls' heads are capable of swiveling through an angle of roughly 270°, easily enabling them to see behind them without relocating the torso.[29] This ability keeps bodily movement at a minimum, thus reduces the amount of sound the owl makes as it waits for its prey. Owls are regarded as having the most frontally placed eyes among all avian groups, which gives them some of the largest binocular fields of vision. Owls are farsighted and cannot focus on objects within a few centimetres of their eyes.[28][30] These mechanisms are only able to function due to the large-sized retinal image.[31] Thus, the primary nocturnal function in the vision of the owl is due to its large posterior nodal distance; retinal image brightness is only maximized to the owl within secondary neural functions.[31] These attributes of the owl cause its nocturnal eyesight to be far superior to that of its average prey.[31]
Owls exhibit specialized hearing functions and ear shapes that also aid in hunting. They are noted for asymmetrical ear placements on the skull in some genera. Owls can have either internal or external ears, both of which are asymmetrical. Asymmetry has not been reported to extend to the middle or internal ear of the owl. Asymmetrical ear placement on the skull allows the owl to pinpoint the location of its prey. This is especially true for strictly nocturnal species such as the barn owlsTyto or Tengmalm's owl.[29] With ears set at different places on its skull, an owl is able to determine the direction from which the sound is coming by the minute difference in time that it takes for the sound waves to penetrate the left and right ears.[32] The owl turns its head until the sound reaches both ears at the same time, at which point it is directly facing the source of the sound. This time difference between ears is about 30 microseconds. Behind the ear openings are modified, dense feathers, densely packed to form a facial ruff, which creates an anterior-facing, concave wall that cups the sound into the ear structure.[33] This facial ruff is poorly defined in some species, and prominent, nearly encircling the face, in other species. The facial disk also acts to direct sound into the ears, and a downward-facing, sharply triangular beak minimizes sound reflection away from the face. The shape of the facial disk is adjustable at will to focus sounds more effectively.[29]
The prominences above a great horned owl's head are commonly mistaken as its ears. This is not the case; they are merely feather tufts. The ears are on the sides of the head in the usual location (in two different locations as described above).
Talons
While the auditory and visual capabilities of the owl allow it to locate and pursue its prey, the talons and beak of the owl do the final work. The owl kills its prey using these talons to crush the skull and knead the body.[29] The crushing power of an owl's talons varies according to prey size and type, and by the size of the owl. The burrowing owl (Athene cunicularia), a small, partly insectivorous owl, has a release force of only 5 N. The largerbarn owl (Tyto alba) needs a force of 30 N to release its prey, and one of the largest owls, thegreat horned owl (Bubo virginianus), needs a force over 130 N to release prey in its talons.[34] An owl's talons, like those of most birds of prey, can seem massive in comparison to the body size outside of flight. TheTasmanian masked owl has some of the proportionally longest talons of any bird of prey; they appear enormous in comparison to the body when fully extended to grasp prey.[35] An owl's claws are sharp and curved. The family Tytonidae has inner and central toes of about equal length, while the family Strigidae has an inner toe that is distinctly shorter than the central one.[34] These different morphologies allow efficiency in capturing prey specific to the different environments they inhabit.
Beak
The beak of the owl is short, curved, and downward-facing, and typically hooked at the tip for gripping and tearing its prey. Once prey is captured, the scissor motion of the top and lower bill is used to tear the tissue and kill. The sharp lower edge of the upper bill works in coordination with the sharp upper edge of the lower bill to deliver this motion. The downward-facing beak allows the owl's field of vision to be clear, as well as directing sound into the ears without deflecting sound waves away from the face.[36]
The coloration of the owl's plumage plays a key role in its ability to sit still and blend into the environment, making it nearly invisible to prey. Owls tend to mimic the coloration and sometimes the texture patterns of their surroundings, the barn owl being an exception. Thesnowy owl (Bubo scandiacus) appears nearly bleach-white in color with a few flecks of black, mimicking their snowy surroundings perfectly, while the speckled brown plumage of thetawny owl (Strix aluco) allows it to lie in wait among the deciduous woodland it prefers for its habitat. Likewise, themottled wood owl (Strix ocellata) displays shades of brown, tan, and black, making the owl nearly invisible in the surrounding trees, especially from behind. Usually, the only tell-tale sign of a perched owl is its vocalizations or its vividly colored eyes.
Most owls arenocturnal, actively hunting their prey in darkness. Several types of owls arecrepuscular—active during the twilight hours of dawn and dusk; one example is thepygmy owl (Glaucidium). A few owls are active during the day, also; examples are theburrowing owl (Speotyto cunicularia) and theshort-eared owl (Asio flammeus).
Much of the owls' hunting strategy depends on stealth and surprise. Owls have at least two adaptations that aid them in achieving stealth. First, the dull coloration of their feathers can render them almost invisible under certain conditions. Secondly, serrated edges on the leading edge of owls'remiges muffle an owl's wing beats, allowing an owl's flight to be practically silent. Some fish-eating owls, for which silence has no evolutionary advantage, lack this adaptation.
An owl's sharpbeak and powerfultalons allow it to kill its prey before swallowing it whole (if it is not too big). Scientists studying the diets of owls are helped by their habit ofregurgitating the indigestible parts of their prey (such as bones, scales, and fur) in the form ofpellets. These "owl pellets" are plentiful and easy to interpret, and are often sold by companies to schools for dissection by students as a lesson in biology and ecology.[37]
Breeding and reproduction
Owl eggs typically have a white color and an almost spherical shape, and range in number from a few to a dozen, depending on species and the particular season; for most, three or four is the more common number. In at least one species, female owls do not mate with the same male for a lifetime. Female burrowing owls commonly travel and find other mates, while the male stays in his territory and mates with other females.[38]
Evolution and systematics
Agreat horned owl (Bubo virginianus) sleeping during daytime in a hollow tree
Cladogram of Telluraves relationships based on Braun & Kimball (2021)[43]
Some 220 to 225 extant species of owls are known, subdivided into two families: 1.true owls or typical owls family (Strigidae) and 2. barn-owls family (Tytonidae). Some entirely extinct families have also been erected based onfossil remains; these differ much from modern owls in being less specialized or specialized in a very different way (such as the terrestrialSophiornithidae). ThePaleocene generaBerruornis andOgygoptynx show that owls were already present as a distinct lineage some 60–57million years ago (Mya), hence, possibly also some 5 million years earlier, at the extinction of the non-aviandinosaurs. This makes them one of the oldest known groups of non-Galloanserae landbirds. The supposed "Cretaceous owls"Bradycneme andHeptasteornis are apparently non-avialanmaniraptors.[44]
During thePaleogene, the Strigiformesradiated intoecological niches now mostly filled by other groups of birds.[clarification needed] The owls as known today, though, evolved their characteristicmorphology andadaptations during that time, too. By the earlyNeogene, the other lineages had been displaced by other bird orders, leaving only barn owls and typical owls. The latter at that time was usually a fairly generic type of (probably earless) owl similar to today's North Americanspotted owl or the Europeantawny owl; the diversity in size andecology found in typical owls today developed only subsequently.
Around the Paleogene-Neogene boundary (some 25 Mya), barn owls were the dominant group of owls in southern Europe and adjacent Asia at least; the distribution of fossil and present-day owl lineages indicates that their decline is contemporary with the evolution of the different major lineages of true owls, which for the most part seems to have taken place in Eurasia. In the Americas, rather, an expansion of immigrant lineages of ancestral typical owls occurred.
The supposed fossilherons"Ardea" perplexa (Middle Miocene of Sansan, France) and"Ardea" lignitum (Late Pliocene of Germany) were more probably owls; the latter was apparently close to the modern genusBubo. Judging from this, the Late Miocene remains from France described as"Ardea" aureliensis should also be restudied.[45] TheMesselasturidae, some of which were initially believed to bebasal Strigiformes, are now generally accepted to be diurnalbirds of prey showing someconvergent evolution toward owls. The taxa often united underStrigogyps[46] were formerly placed in part with the owls, specifically the Sophiornithidae; they appear to beAmeghinornithidae instead.[47][48][49]
For fossil species andpaleosubspecies of extanttaxa, see the genus and species articles. For a full list of extant and recently extinct owls, see the articleList of owl species.
Unresolved and basal forms (all fossil)
Berruornis (Late Paleocene of France) basal? Sophornithidae?
Strigiformes gen. et sp. indet. (Late Paleocene of Zhylga, Kazakhstan)[50]
"Asio" pygmaeus –fossil (Early Pliocene of Odesa, Ukraine)
Strigidae gen. et sp. indet. UMMP V31030 (Late Pliocene) –Strix/Bubo?
the Ibizan owl, Strigidaegen. et sp. indet. –prehistoric[54]
Symbolism and mythology
Among theKikuyu ofKenya, it was believed that owls were harbingers of death. If one saw an owl or heard its hoot, someone was going to die. In general, owls are viewed as harbingers of bad luck, ill health, or death. The belief is widespread even today.[55]
InHinduism, an owl is thevahana (mount) of the goddessLakshmi, especially in the eastern region of India.[56] Owls are considered a symbol of wealth, prosperity, wisdom, good luck, and fortune. This is the reason why Owls are seen with Lakshmi, who is also the goddess of fortune, wealth, and prosperity. At the same time, owls are also associated with evil times in Hinduism. At times,Chamunda (fearsome form ofChandi) is depicted seated on an owl, hervahana (mount or vehicle). Hindus believe that owls are messengers of death.[57][better source needed] InChina, owls were traditionally considered to be omens of evil or misfortune. InJapan, owls are regarded as lucky,[58] although in ancient times they were associated with death.[59]
In Sumerian, Akkadian, and Babylonian culture, the owl was associated withLilith.[60]
In medieval Europe, the owl was often used with more negative symbolism. The bird was associated with darkness, uncleanliness, and mourning. The bird was linked to thebubonic plague. Owls were even used as anti-Semetic caricatures of Jews.[63][64]
from the earliest period it has been considered a bird of ill-omen," andPliny tells us how, on one occasion, evenRome itself underwent alustration, because one of them strayed into the Capitol. He represents it also as a funereal bird, a monster of the night, the very abomination of humankind.Virgil describes its death howl from the top of the temple by night, a circumstance introduced as a precursor ofDido's death.Ovid, too, constantly speaks of this bird's presence as an evil omen; and indeed the same notions respecting it may be found among the writings of most of the ancient poets.[65]
Native American cultures
People often allude to the reputation of owls as bearers of supernatural danger when they tell misbehaving children, "the owls will get you",[66] and in most Native American folklore, owls are a symbol of death.
According to theApache andSeminole tribes, hearing owls hooting is considered the subject of numerous "bogeyman" stories told to warn children to remain indoors at night or not to cry too much, otherwise the owl may carry them away.[67][68] In some tribal legends, owls are associated withspirits of the dead, and the bony circles around an owl's eyes are said to comprise the fingernails of apparitional humans. Sometimes owls are said to carry messages from beyond the grave or deliver supernatural warnings to people who have broken tribal taboos.[69]
TheAztecs and theMaya, along with other natives ofMesoamerica, considered the owl a symbol of death and destruction. In fact, the Aztec god of death,Mictlantecuhtli, was often depicted with owls.[70] There is an old saying inMexico that is still in use:[71]Cuando el tecolote canta, el indio muere ("When the owl cries/sings, the Indian dies"). ThePopol Vuh, a Mayan religious text, describes owls as messengers ofXibalba (the Mayan "Place of Fright").[72]
The belief that owls are messengers and harbingers of the dark powers is also found among theHočągara (Winnebago) of Wisconsin.[73] When in earlier days the Hočągara committed the sin of killing enemies while they were within the sanctuary of the chief's lodge, an owl appeared and spoke to them in the voice of a human, saying, "From now on, the Hočągara will have no luck." This marked the beginning of the decline of their tribe.[74] An owl appeared toGlory of the Morning, the only female chief of the Hočąk nation, and uttered her name. Soon after, she died.[75][76]
According to the culture of theHopi, aUto-Aztec tribe, taboos surround owls, which are associated with sorcery and other evils.[citation needed]
TheOjibwe tribes, as well as theirAboriginal Canadian counterparts, used an owl as a symbol for both evil and death. In addition, they used owls as a symbol of very high status of spiritual leaders of their spirituality.[77]
ThePawnee tribes viewed owls as the symbol of protection from any danger within their realms.[77]
ThePuebloan peoples associated owls with Skeleton Man, the god of death and the spirit of fertility.[77]
TheYakama tribes use an owl as a totem, to guide where and how forests and natural resources are useful with management.[77]
Rodent control
A purpose-built owl-house or owlery at a farm nearMorton on the Hill, England (2006)
Encouraging natural predators to control rodent population is a natural form of pest control, along with excluding food sources for rodents. Placing anest box for owls on a property can help control rodent populations (one family of hungry barn owls can consume more than 3,000 rodents in a nesting season) while maintaining the naturally balanced food chain.[78]
Almost all owls are listed in Appendix II of the internationalCITES treaty (the Convention on Illegal Trade in Endangered Species of Wild Fauna and Flora) with four species listed in Appendix I. Although owls have long been hunted, a 2008 news story from Malaysia indicates that the magnitude of owlpoaching may be on the rise. In November 2008,TRAFFIC reported the seizure of 900 plucked and "oven-ready" owls in PeninsularMalaysia. Said Chris Shepherd, Senior Programme Officer for TRAFFIC's Southeast Asia office, "This is the first time we know of where 'ready-prepared' owls have been seized in Malaysia, and it may mark the start of a new trend in wild meat from the region. We will be monitoring developments closely." TRAFFIC commended the Department of Wildlife and National Parks in Malaysia for the raid that exposed the huge haul of owls. Included in the seizure were dead and plucked barn owls, spotted wood owls, crested serpent eagles, barred eagles, and brown wood owls, as well as 7,000 live lizards.[81]
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