Tawny Owl

The **tawny owl** or **brown owl** (*Strix aluco*) is a stocky, medium-sized owl commonly found in woodlands across much of Eurasia. Its underparts are pale with dark streaks, and the upperparts are either brown or grey. Several of the eleven recognised subspecies have both variants. The nest is typically in a tree hole where it can protect its eggs and young against potential predators. This owl is non-migratory and highly territorial. Many young birds starve if they cannot find a vacant territory once parental care ceases.

This nocturnal bird of prey hunts mainly rodents, usually by dropping from a perch to seize its prey, which it swallows whole; in more urban areas its diet includes a higher proportion of birds. Vision and hearing adaptations and silent flight aid its night hunting. The tawny owl is capable of catching smaller owls, but is itself vulnerable to the eagle owl or northern goshawk.

Although many people believe this owl has exceptional night vision, its retina is no more sensitive than a human's and its asymmetrically placed ears are key to its hunting by giving it excellent directional hearing. Its nocturnal habits and eerie, easily imitated call, have led to a mythical association of the tawny owl with bad luck and death.

## Description

The tawny owl is a robust bird, 37–46 cm (15–18 in) in length, with an 81–105 cm (32–41 in) wingspan. Weight can range from 385 to 800 g (0.849 to 1.764 lb). 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. 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; great grey, eagle and Ural owls 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 (overlap 30–50%). The tawny owl's retina has about 56,000 light-sensitive rod cells per square millimetre (36 million per square inch); 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 cellshave 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 like 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 pre-aural flap).

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* (Act 5, Scene 2) 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.

Check out Wikipedia for more details!