Tawny owl
The tawny owl, also called the brown owl, is a stocky, medium-sized owl in the family Strigidae. It is commonly found in woodlands across Europe, as well as western Siberia, and has seven recognized subspecies. The tawny owl's underparts are pale with dark streaks, whilst its upper body may be either brown or grey. The tawny owl typically makes its nest in a tree hole where it can protect its eggs and young against potential predators. It is non-migratory and highly territorial: as a result, when young birds grow up and leave the parental nest, if they cannot find a vacant territory to claim as their own, they will often starve.
The tawny owl is a nocturnal bird of prey. It is able to hunt successfully at night because of its vision, hearing adaptations and ability to fly silently. It usually hunts by dropping suddenly from a perch and seizing its prey, which it swallows whole. Its typical prey are rodents, although in urbanized areas its diet includes a higher proportion of birds. It also sometimes catches smaller owls, and is itself sometimes hunted by the eagle owl and the Eurasian goshawk.
Its retina is no more sensitive than a human's. Its directional hearing skill is more important to its hunting success: its ears are asymmetrically placed, which enables it to more precisely pinpoint the location from which a sound originates.
The tawny owl holds a place in human folklore: because it is active at night and has what many humans experience as a haunting call, people have traditionally associated it with bad omens and death. Not all owl species make a hooting sound. The double hoot, the tawny owl's prototypical call, is a call and response between a male and a female.
Description
The tawny owl is a robust bird, in length, with an wingspan. Weight can range from. Its large rounded head lacks ear tufts, and the facial disc surrounding the dark brown eyes is usually rather plain. The nominate race has two morphs which differ in their plumage colour, one form having rufous brown upperparts and the other greyish brown, although intermediates also occur. The underparts of both morphs are whitish and streaked with brown. Feathers are moulted gradually between June and December. This species is sexually dimorphic; the female is much larger than the male, 5% longer and more than 25% heavier.The tawny owl flies with long glides on rounded wings, less undulating and with fewer wingbeats than other Eurasian owls, and typically at a greater height. The flight of the tawny owl is rather heavy and slow, particularly at takeoff, though the bird can attain a top flight speed of around 50 mph. As with most owls, its flight is silent because of its feathers' soft, furry upper surfaces and a fringe on the leading edge of the outer primaries. Its size, squat shape and broad wings distinguish it from other owls found within its range; the great grey owl, Eurasian eagle-owl and Ural owl are similar in shape, but much larger.
An owl's eyes are placed at the front of the head and have a field overlap of 50–70%, giving it better binocular vision than diurnal birds of prey. The tawny owl's retina has about 56,000 light-sensitive rod cells per square millimetre ; although earlier claims that it could see in the infrared part of the spectrum have been dismissed, it is still often said to have eyesight 10 to 100 times better than humans in low-light conditions. However, the experimental basis for this claim is probably inaccurate by at least a factor of 10. The owl's actual visual acuity is only slightly greater than that of humans, and any increased sensitivity is due to optical factors rather than to greater retinal sensitivity; both humans and owl have reached the limit of resolution for the retinas of terrestrial vertebrates.
Adaptations to night vision include the large size of the eye, its tubular shape, large numbers of closely packed retinal rods, and an absence of cone cells, since rod cells have superior light sensitivity. There are few coloured oil drops, which would reduce the light intensity. Unlike diurnal birds of prey, owls normally have only one fovea, and that is poorly developed except in daytime hunters such as the short-eared owl.
Hearing is important for a nocturnal bird of prey, and as with other owls, the tawny owl's two ear openings differ in structure and are asymmetrically placed to improve directional hearing. A passage through the skull links the eardrums, and small differences in the time of arrival of a sound at each ear enables its source to be pinpointed. The left ear opening is higher on the head than the larger right ear and tilts downward, improving sensitivity to sounds from below. Both ear openings are hidden under the facial disk feathers, which are structurally specialized to be transparent to sound, and are supported by a movable fold of skin.
The internal structure of the ear, which has large numbers of auditory neurons, gives an improved ability to detect low-frequency sounds at a distance, which could include rustling made by prey moving in vegetation. The tawny owl's hearing is ten times better than a human's, and it can hunt using this sense alone in the dark of a woodland on an overcast night, but the patter of raindrops makes it difficult to detect faint sounds, and prolonged wet weather can lead to starvation if the owl cannot hunt effectively.
The commonly heard female contact call is a shrill, kew-wick but the male has a quavering advertising song hoo...ho, ho, hoo-hoo-hoo-hoo. William Shakespeare used this owl's song in Love's Labour's Lost as "Then nightly sings the staring owl, Tu-whit; Tu-who, a merry note, While greasy Joan doth keel the pot", but this stereotypical call is actually a duet, with the female making the kew-wick sound, and the male responding hooo. The call is easily imitated by blowing into cupped hands through slightly parted thumbs, and a study in Cambridgeshire found that this mimicry produced a response from the owl within 30 minutes in 94% of trials. A male's response to a broadcast song appears to be indicative of his health and vigour; owls with higher blood parasite loads use fewer high frequencies and a more limited range of frequencies in their responses to an apparent intruder. The vocal activity of tawny owls depends on sex, annual cycle stage and weather, with males being more vocal than females year-round, with peak vocal activity during incubation and post-breeding.
Geographical variation
Although both colour morphs occur in much of the European range, brown birds predominate in the more humid climate of western Europe, with the grey morph becoming more common further east; in the northernmost regions, all the owls are a cold-grey colour. The Siberian and Scandinavian subspecies are 12% larger and 40% heavier, and have 13% longer wings than western European birds, in accordance with Bergmann's rule which predicts that northern forms will typically be bigger than their southern counterparts.The plumage colour is genetically controlled, and studies in Finland and Italy indicate that grey-morph tawny owls have more reproductive success, better immune resistance, and fewer parasites than brown birds. Although this might suggest that eventually the brown morph could disappear, the owls show no colour preference when choosing a mate, so the selection pressure in favour of the grey morph is reduced. There are also environmental factors involved. The Italian study showed that brown-morph birds were found in denser woodland, and in Finland, Gloger's rule would suggest that paler birds would in any case predominate in the colder climate.
Taxonomy
The species was given its current scientific name Strix aluco by Carl Linnaeus in the tenth edition of his Systema Naturae in 1758. The binomial derives from the Greek strix "owl" and Italian allocco "tawny owl".The tawny owl is a member of the wood-owl genus Strix, part of the typical owl family Strigidae, which contains all species of owl other than the barn owls. Within its genus, the tawny owl's closest relatives are Hume's owl, the Himalayan owl, its larger northern neighbour, the Ural owl, and the North American barred owl. The Early–Middle Pleistocene S. intermedia is sometimes considered a paleosubspecies of the tawny owl, which would make it that species' immediate ancestor.
The tawny owl subspecies are often poorly differentiated, and may be at a flexible stage of subspecies formation with features related to the ambient temperature, the colour tone of the local habitat, and the size of available prey. Consequently, various authors have historically described between 10 and 15 subspecies. The seven currently recognised subspecies are listed below.
| Subspecies | Range | Described by |
| S. a. aluco | north and central Europe from Scandinavia to the Mediterranean and Black Sea | Linnaeus, 1758 |
| S. a. biddulphi | northwest India and Pakistan | Scully, 1881 |
| S. a. harmsi | Kazakhstan, Uzbekistan and Kyrgyzstan | |
| S. a. sanctinicolai | west Iran, northeast Iraq | |
| S. a. siberiae | central Russia from Urals to west Siberia | Dementiev, 1934 |
| S. a. sylvatica | west and southern Europe, west Turkey | Shaw, 1809 |
| S. a. willkonskii | northeast Turkey and northwest Iran to Turkmenistan |
Distribution and habitat
The tawny owl is non-migratory and has a distribution stretching discontinuously across temperate Europe, from Great Britain and the Iberian Peninsula eastwards to western Siberia. It is absent from Ireland - probably because of competition from the long-eared owl - and only a rare vagrant to the Balearic and Canary Islands. In the Himalayas and East Asia it is replaced by the Himalayan owl and in northwest Africa it is replaced by the closely related Maghreb owl.This species is found in deciduous and mixed forests, and sometimes mature conifer plantations, preferring locations with access to water. Cemeteries, gardens and parks have allowed it to spread into urban areas, including central London. Although tawny owls occur in urban environments, especially those with natural forests and wooded habitat patches, they are less likely to occur at sites with high noise levels at night. The tawny owl is mainly a lowland bird in the colder parts of its range, but breeds to in Scotland, in the Alps, in Turkey, and up to in Myanmar.
The tawny owl has a geographical range of at least 10 million km2 and a large population including an estimated 970,000–2,000,000 individuals in Europe alone. Population trends have not been quantified, but there is evidence of an overall increase. This owl is not believed to meet the IUCN Red List criterion of declining more than 30% in ten years or three generations and is therefore evaluated as being of least concern. In the UK it is on the RSPB Amber List of Concern. This species has expanded its range in Belgium, the Netherlands, Norway and Ukraine, and populations are stable or increasing in most European countries. Declines have occurred in Finland, Estonia, Italy and Albania. Tawny owls are listed in Appendix II of the Convention on International Trade in Endangered Species meaning international trade is regulated.