Strix aluco

Geographic Range

Strix aluco can be found across the Palearctic Region from the Iberian Peninsula to as far east as China and Korea and south to Iran and the Himalayan mountain range. It is native to the British Isles and is commonly found there, except for in northern Scotland. Tawny owls are not found in Ireland. Though they are nonmigratory, they have been found wintering in Morocco and, rarely, in Egypt and the Balearic and Canary Islands.

There are 11 known subspecies of tawny owls: Strix aluco aluco in northern and central Europe from Scandinavia to the Mediterranean and Black Sea, Strix aluco mauritanica , from northwestern Africa from Morocco to Tunisia and Mauritania, Strix aluco sylvatica , in western Europe including Britain, Strix aluco siberiae in Central Russia from the Urals to western Siberia, Strix aluco sanctinicola in western Iran and northeast Iraq, Strix aluco wilkonskii from Palestine to Northern Iran and the Caucasus, Strix aluco harmsi , in Turkmenistan, Strix aluco bidulphi , in northwestern India and Pakistan, Strix aluco nivicola , from Nepal to southeastern China and south to northern Myanmar and Thailand, Strix aluco yamadae , in Taiwan, Strix aluco ma , in northeastern China and Korea.

• Biogeographic Regions
• palearctic palearctic
  • native native

Habitat

Tawny owls live in open, deciduous, or mixed forest or woodland, agricultural areas with trees, parks, cemeteries, and large gardens, preferring locations with access to water. While sometimes found in mature conifer forests as well, they prefer mature broadleaf trees, such as ancient oaks with large holes for nesting. Tawny owls are frequently found near human habitation (for example, in central London) and in the winter can be found nesting in abandoned buildings and rock cavities. They are lowland birds in the colder parts of their range, but can breed at higher altitudes.

• Habitat Regions
• temperate temperate
• terrestrial terrestrial

• Terrestrial Biomes
• forest forest

• Other Habitat Features
• urban urban
• suburban suburban
• agricultural agricultural

Physical Description

Tawny owls are medium-sized and compact. They have large, rounded heads with no ear tufts. These owls exhibit geographic variation in color. They can be rufous-brown; greyish-brown with the mottled plumage, finely streaked, and with dark vermiculation (more commonly seen in the eastern part of the bird's range); or lighter grey and white (in the northernmost parts of their distribution). South and east Asian subspecies have barred undersides instead of striped and have fine facial disk lines. Siberian and Scandinavian subspecies are 12% larger and 40% heavier than western European tawny owls, with 13% longer wings. Females are more than 25% heavier and 5% longer than males.

In all subspecies the facial discs are usually plain, with pale whitish crown-stripes or extra "eyebrows" that add to the owl's kindly expression. The eyes are black, which prevents them from being confused with the yellow eyes of long-eared owls , which can appear black in headlights at night due to their large pupils. The shoulder feathers are lined with white spots and there are no pale markings on the inner primary feathers. The tail is finely barred. Young owls are more pale than adults. When flying these owls have quick wingbeats and glide long and straight on broad wings. Great grey owls , eagle owls and Ural owls resemble tawny owls in shape, but are much larger.

• Other Physical Features
• endothermic endothermic
• homoiothermic
• bilateral symmetry bilateral symmetry

• Sexual Dimorphism
• female larger

Reproduction

Tawny owls reach sexual maturity after one year and most form monogamous pairs for life, although some males exhibit polygynous behavior, mating with more than one female. In October or November, male owls establish territories while females find nesting holes. At this point in the year, males and females roost separately. The pair defend their territory year-round with minor changes to boundaries each year. As winter approaches, territories are finalized and pre-breeding behavior begins with males and females roosting together. This is the time for courtship feeding, which is centered around the future nesting site. A male perches near a female, swaying from side to side and up and down. He raises each wing alternatively, then together, and puffs out his feathers to appear larger. He slides closer to her on the branch, grunting softly and clapping his wings. In response, the female screeches and makes a variety of noises. She may also puff her feathers out and quiver.

Some mating behavior differs between rufous and greyish-brown morphs. Greyish-brown females breed less often than rufous females and may not breed every year, although survival probability between morphs is similar. They do, however, produce offspring of higher quality and fitness than those of rufous females. It would seem that color polymorphism is an indicator of individual quality, with grey morphs having a fitness advantage.

• Mating System
• monogamous monogamous
• polygynous polygynous

The breeding season is from January to July. The nesting site is usually a hole in a tree, although they will take over abandoned nests of other birds in trees and cliffs, squirrel dreys, holes in old buildings, and artificial nest boxes. In the southern ranges nesting begins in February, and in the northern ranges nesting is in mid-March. Clutches are laid mid-March to early May, usually consisting of 2 to 3 eggs. Sometimes as few as 1 egg or as many as 9 are laid. The eggs are white and are 48 x 39 mm in size, weighing 39 g, of which 7% is shell. Females incubate eggs for about 30 days until they hatch into downy, altricial chicks. Chicks fledge in another 35 to 39 days.

The success of broods depends heavily on the ages of parents. Older birds have the ability to deliver a greater mass of alternate prey at a higher frequency and productivity than younger birds when preferred prey is scarce. Young parents pay a higher reproductive cost as they have lower ability to exploit alternative food sources and provide resources to their brood due to their poorer body condition and the need to tend to their own survival.

• Key Reproductive Features
• iteroparous iteroparous
• seasonal breeding seasonal breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual sexual
• oviparous oviparous

Tawny owls invest heavily in their chicks. Once the eggs have hatched, males bring food to the nest. Females leaves the nest only to hunt once the downy, altricial chicks are several days old and for most of the time remains close to her brood. Even after fledging, juveniles depend on their parents for food for 2 to 3 months after leaving the nest. After this point, around August to November, young owls must leave to find their own hunting territories and fend for themselves or risk starving to death.

• Parental Investment
• altricial altricial
• male parental care male parental care
• female parental care female parental care
• pre-fertilization
  • provisioning
  • protecting
    • male
• pre-hatching/birth
  • provisioning
    • male
    • female
  • protecting
    • male
    • female
• pre-weaning/fledging
  • provisioning
    • male
    • female
  • protecting
    • male
    • female
• pre-independence
  • provisioning
    • male
    • female
  • protecting
    • male
    • female
• post-independence association with parents
• extended period of juvenile learning

Lifespan/Longevity

The typical lifespan of Strix aluco in the wild is 4 years. The oldest wild tawny owl ever recorded lived 21 years and 5 months. Captive birds can live over 27 years. The adult annual survival rate is 73.8% and the juvenile annual survival rate is 30.1%. If juveniles fail to find a vacant territory after leaving the nest they usually starve to death.

In an experiment conducted in 2008 to establish the post-release survival rates of hand-reared tawny owls, researchers discovered that 66% survived longer than six weeks. Only 39% survived over a year and only one bird survived longer than the average lifespan of wild tawny owls. This has been deemed a sufficient percentage to justify the rehabilitation of juvenile owls to be released in the wild by the Royal Society for the Prevention of Cruelty to Animals.

Behavior

Tawny birds are non-migratory. They are primarily nocturnal, although they can be active during the day. The flight of tawny owls is characterized by relatively quick wingbeats and marked agility, maneuvering in and around the trees of their woodland habitat. In open spaces, these owls glide or hover on extended wings. Tawny owls have a variety of calls for communicating to others of the species. There are specific calls for males bringing prey home to females, for parents bringing prey home to young, as well as mating and territory disputes. These owls are highly territorial and establish their territories with screeching calls, threatening behavior, or in fights mid-flight. Tawny owls defend their nest and young fiercely, even in the daytime, striking with sharp talons at the intruder's head and eyes. Their silent flight give them the ability to attack without notice. Tawny owl mothers aggressively defending their broods, even against humans and domestic animals; at least two cases have been reported in Great Britain of people losing an eye to a tawny owl attack. This strong territorial sense forces juveniles to find territory away from their home range once they are independent.

• Key Behaviors
• arboreal arboreal
• flies
• glides
• diurnal diurnal
• nocturnal nocturnal
• crepuscular crepuscular
• motile motile
• sedentary sedentary
• solitary solitary
• territorial territorial

Home Range

Territory size of tawny owls depends on the habitat and availability of preferred prey. Territories can range from 30 acres in heavy woodland to 175 acres average in a beechwood forest with less vegetation. In Norway, where prey density is low, territories range about 255 acres.

Communication and Perception

Tawny owls are notably vocal. The most commonly known call is "ke-WICK" repeated shrilly, with frightening variations used as alarm or to display aggression. The song is mournful and ocarina-like, and is easily mimiced by blowing into one's hands. It begins with a long drawn-out note falling in pitch, followed by a short pause and a series of quick, shivering notes, to end with another falling note: "HOOOOuh.......ho, ho'ho'ho'HOOOOOOuh." The female exhibits a hoarse and wailing version of this song. The call used in mating is called the 'xylophone trill': "'o'o'o'o'o'o'o'o'" and it used by both sexes. Young owls beg for food by squeaking: "PSEE-ep."

Tawny owl eyes are at the front of the head, with a field overlap of 50 to 70%. This overlap enables greater binocular vision than birds that hunt during the day. The increased visual acuity of the owl, while not much greater than that in humans, is due to optical factors such as the shape and size of the eye itself rather than retinal sensitivity.

Their two ear openings have different structures and are located asymmetrically on the head for directional hearing. Hearing is a crucial sense for a nocturnal hunter. These owls can use the minute differences in the time of arrival of sound at each ear to locate the source. The left ear tilts downward to catch sounds coming from below. The ears are underneath the facial disk feathers, which are transparent to sound and supported by the movable pre-aural flap. The hearing of Strix aluco is ten times better than that of humans, although this acute hearing can be easily interfered with by the sound of rainfall. Prolonged rain can bring about an inability to hunt and inevitable starvation.

• Communication Channels
• acoustic acoustic

• Perception Channels
• visual visual
• tactile tactile
• acoustic acoustic
• chemical chemical

Food Habits

Tawny owls prey on a variety of animals, ranging from mostly woodland rodents to other small mammals, amphibians, birds, beetles, and worms. Birds represent a larger percentage of their diet in urban areas, including mallards and kittiwakes .

Tawny owls hunt primarily between dusk and dawn. They perch and watch for prey, then use silent gliding flight to catch their victim on the ground, extending the wings to cover the prey and killing it with feet and claws. Occasionally they may use the beak to deliver a blow to the base of the victim's neck. Tawny owls have also been reported to beat their wings to flush smaller birds out of hiding and into flight and then take aerial pursuit. They also fly over grassland, marshland, or bushes looking for bats or incubating birds to pluck from their roosting perches and nests. Mother owls may hunt during the day to feed their young.

Owls swallow prey whole, parts that are indigestible are later regurgitated in the form of pellets. These medium-sized pellets, usually grey in color, contain mostly rodent fur and bones and are found around trees where owls nest. Owl pellets can reveal much about the dietary habits of the bird in question.

• Primary Diet
• carnivore carnivore
  • eats terrestrial vertebrates

• Animal Foods
• birds
• mammals
• amphibians
• fish
• insects
• terrestrial worms

Predation

Tawny owls are preyed on by larger birds such as northern goshawks , common buzzards , and other species of owls, such as Ural and eagle owls. Pine martens are known to raid owl nests, and human supplying of artificial nesting boxes in urban areas makes owl fledglings easier for predators to find. Eurasian jackdaws sometimes build nests on top of female tawny owls and their nests, killing both adult and chicks.

A 2005 study revealed red foxes , stone martens , and other mammalian predators as a prime factor in the mortality rates of fledgling tawny owls, 36% of fledglings die within the first 55 days of leaving the nest. Predation increases by over 44% as the year progresses, providing selection pressure for early breeding in Strix aluco . Avian predators such as raptors pose the second biggest threat to juveniles.

• Anti-predator Adaptations
• cryptic cryptic

Ecosystem Roles

Tawny owls are formidable predators in all of the habitats they occupy. Smaller owls cannot coexist with tawny owls, which will prey on them as well as compete for prey. Tawny owls also displace barn owls in urban areas, taking over their nests in buildings. As predators, tawny owls help control populations of natural prey.

Tawny owls are hosts to several kind of blood parasites, including Leucocytozoon , Haemoproteus , and Trypanosoma . These parasites negatively affect the fitness of the afflicted bird and breeding behaviors within the morphs. The rufous morph is more susceptible to these parasites because of their open habitat and greater exposure.

Economic Importance for Humans: Positive

Strix aluco has no major positive impacts on humans aside from preying upon small animals that may be considered agricultural pests.

• Positive Impacts
• controls pest population

Economic Importance for Humans: Negative

Because of their fierce protectiveness of nests, any potential intruder may be attacked, including humans. Humans have been attacked by Strix aluco , even without apparent provocation.

• Negative Impacts
• injures humans

Conservation Status

Tawny owls are of least concern on the IUCN list and populations are stable. They are considered highly adaptable and are locally very common.

Additional Links

Encyclopedia of Life

Contributors

Katrina Diaz (author), University of Michigan-Ann Arbor, Phil Myers (editor), University of Michigan-Ann Arbor, Tanya Dewey (editor), University of Michigan-Ann Arbor.

References

Arlott, N. 2009. Birds of Europe, Russia, China, and Japan : Non-passerines, Loons to Woodpeckers. . Princeton, N.J.: Princeton University Press.

Brown, R., J. Ferguson, M. Lawrence, D. Lees. 1987. Tracks and Signs of the Birds of Britain and Europe (Helm Identification Guides) . London: Christopher Helm Publishers Ltd.

Burton, R. 1985. Bird Behaviour . London: Granada Publishing.

Galeotti, P., R. Sacchi. 2003. Differential parasitaemia in the tawny owl (Strix aluco): effects of colour morph and habitat. Journal of Zoology , 261: 91-99. Accessed April 15, 2010 at http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=169673 .

Lack, P. 1986. The Atlas of Wintering Birds in Britain and Ireland . Calton: T. & A. D. Poyser Ltd..

Leighton, K., D. Chilvers, A. Charles, A. Kelly. 2008. Post-release survival of hand-reared tawny owls (Strix aluco) based on radio-tracking and leg-band return data. Animal Welfare , 17: 207-214.

Lewis, D. 2006. "OwlPages.com" (On-line). Eurasian Tawny Owl - Strix aluco. Accessed April 15, 2010 at http://www.owlpages.com/owls.php?genus=Strix&species=aluco .

Martin, G. 1977. Absolute visual threshold and scotopic spectral sensitivity in the tawny owl Strix aluco. Nature , 268: 636-638.

Ramsay, R. 1923. Guide to the Birds of Europe and North Africa . London: Gurney and Jackson.

Roulin, A., B. Dcuret, P. Ravussin, R. Altwegg. 2003. Female colour polymorphism covaries with reproductive strategies in the tawny owl Strix aluco. Journal of Avian Biology , 34: 393-401.

Sasvária, L., Z. Hegyib, T. Csörgõa, I. Hahnc. 2000. Age-dependent diet change, parental care and reproductive cost in tawny owls Strix aluco. Acta Oecologica , 21: 267-275.

Snow, D., C. Perrins. 1998. The Birds of the Western Palearctic . Oxford: Oxford University Press.

Sunde, P. 2005. Predators control post-fledging mortality in tawny owls, Strix aluco. Oikos , 110: 461-472.

Svensson, L., D. Zetterström. 1999. Birds of Europe . Princeton, N.J.: Princeton University Press.

Voous, K. 1988. Owls of the Northern Hemisphere . London: Collins.

British Trust for Ornithology (BTO). 2010. "Tawny Owl Strix aluco [Linnaeus, 1758]" (On-line). BirdFacts. Accessed March 18, 2010 at http://blx1.bto.org/birdfacts/results/bob7610.htm .

World Owl Trust. 2005. "Tawny Owl Strix aluco" (On-line). World Owl Trust. Accessed March 18, 2010 at http://www.owls.org/Species/strix/tawny_owl.htm .