The Tawny owl is probably Britain’s most common and widespread owl species. Its characteristic “twit-twoo”, which is actually a repeated succession of two different calls (“kee-wick” and “hoo-hooo”), is frequently heard in our woodlands, parks and gardens, particularly during the winter. Despite being often-heard, it is rarely seen, preferring to stay in tree hollows or tight against tree trunks during the daytime away from the attention of mobbing birds.
That which follows is a summary of Tawny owl natural history. The detailed article for this species will follow in due course.
The Tawny owl at a glance
Eleven subspecies described, probably only eight valid; British subspecies Strix aluco sylvatica lighter in colour and heavier than the type (S. a. aluco) race.
Size: Reasonably small owl, with adults standing 30-46cm (12-18 in.) tall and weighing between 380-660g (13-23 oz.). It has a wingspan of 81–96 cm (32–38 in.).
Colour/Appearance: Reddish-brown above, paler underside. Dark feathers around face; dark eyes. Two or three colour morphs found in Britain — brown, grey and rufous (red) — with some intermediates described. Some authorities do not separate rufous and brown morphs. Some indication that different colour morphs may have differing survival probabilities associated with risk of predation, metabolism and immune system strength.
Distribution: Most common owl of central Europe. Found throughout UK year-round, but apparently absent from Ireland; rare visitors to islands (e.g. Isle of Wight). RSPB estimated breeding population of 19,400 birds.
Longevity: Wild birds generally don’t exceed 5 yrs old, although an 18 yr old wild individual has been documented. In captivity, 27 yrs has been recorded.
Sexing: Sexually dimorphic in size, with females larger than males. Females are 20-40% heavier, and have a wingspan 5-10% larger, than males. Likely that males and females can be distinguished based on call types, but some authorities dispute this (see: Behaviour and Sociality).
Activity: Hunt nocturnally, but may be active during the daytime. Can be found sitting in trees or at roost sites during the day. Despite being common, they are rarely seen, although daytime calling is relatively common in some areas.
Territory/Habitat: Established pairs defend territory from other owls. Predominantly a bird of mixed and deciduous woodland, although is highly adaptable and can be found in parks and larger gardens, even in the middle of large cities. Radio-tracked birds show preference for deciduous and mature conifer forest. Often roosts in holes in trees or among ivy; may be found sitting against the trunk of large trees or on main branches. Can survive in small (eight hectares) areas rich in food, although this is generally considered too small to permit breeding. Breeding territories vary according to resources, from an average of 12 ha (one-tenth sq-kilometre or one-twentieth sq-mile) in deciduous woodland, to 37 ha in mixed farmland and 46 ha (half sq-kilometre or one-fifth sq-mile) in mature conifer forests. In areas of very low food supply, such as in marshy areas, territories may cover more than 100 ha (one sq-km or almost half sq-mile).
Diet: Small mammals (especially rodents in rural areas), small birds (esp. in urban areas) and even other raptors, amphibians, reptiles (e.g. slowworms and grass snakes), fish (e.g. goldfish from garden ponds) and invertebrates (e.g. insects and worms). Known to catch birds, bats and insects in flight. They will also take carrion if available. Surplus food may be cached in trees. Food swallowed whole and indigestible parts regurgitated as pellet.
Reproduction: Generally monogamous (polyandry is known), with a life-long pair-bond that is maintained throughout the year. Pair-bond strengthened during the autumn and winter with prolific calling and courtship feeding (male presents food to female). Have first clutch at around two years old. Average clutch is two-or-three eggs (up to nine have been recorded in Britain), laid at roughly two-day intervals from late March onwards (January clutches are known). Woodland/farmland birds tend to lay March/April, while city birds often nest earlier owing to increased food availability; eggs in February. Eggs take about four weeks to hatch and owlets are brooded for around 20 days. Leave the nest at around 25 days old, but hang around and aren’t fully fledged until they’re five or six weeks old. Chicks may be fed by parents until ca. 3 months old. Young disperse in autumn.
Behaviour and Sociality: Form pairs, but otherwise antisocial. If mobbed, these birds make no attempt to fight, instead remaining still until their antagonists get bored and move on. Territory crucial for survival and those failing to secure territory often starve. Immature owls may wander long (up to 745km / 503 mi.) distances. Calls with classic ‘ke-wick-hoo-hoo’; calls probably sexually specific as only males seem to make deep, powerful ‘hoo-hoooo’ calls, while females make strong ‘ke-wick’ and more plaintive ‘hoo-hooo’. Territories and pair-bonds reaffirmed during autumn and early winter, resulting in much calling.
Threats: No major threats and not classified as a threatened species by the ICUN. Vulnerable to predation from Eagle owls (Bubo bubo), Northern goshawks (Accipiter gentilis) and Red foxes (Vulpes vulpes). Other owls (e.g. Barn owls) known susceptible to rodent poisons (rodenticides) used to kill agricultural pests; Tawny probably also vulnerable. Recent data from Scandinavia suggest that brown morphs suffer higher mortality than grey morphs during harsh winters, possibly because they’re more obvious to predators against snow. Some biochemical data to indicate that brown morphs have higher metabolisms and weaker immune systems than grey morphs and may thus be less adaptable.
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New Holland Publishers -- 2003 -- ISBN: 978-1845171131
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