Tawny owl

The tawny owl (Strix aluco), 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 (in several subspecies, individuals may be of both colours). 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.

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, 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 (Strix nebulosa), Eurasian eagle-owl (Bubo bubo) and Ural owl (Strix uralensis) 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 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.