Barn owls are unquestionably one of the most instantly recognisable birds in Britain and were once a common sight in our countryside. In recent years, however, the barn owl has suffered a decline in numbers to the point that seeing one is an increasingly rare privilege across much of the UK. Habitat loss, increasing volumes of road traffic, and the use of rodenticide poisons to reduce populations of the rats and mice on which they feed are considered the main causes behind the owl's decline, although climate change may also be having a detrimental impact. Encouragingly, the last few years has seen many landowners becoming much more aware of the needs of this species; hopefully with a positive outcome for the species.
That which follows is a summary of barn owl natural history.
The Barn owl at a glance
At least 31 subspecies have been proposed globally, based largely on colouration, but DNA data data suggest probably no more than 10 are valid. The type subspecies, Tyto alba alba, is the one found throughout the UK.
Size: Smaller than many people imagine, standing 34 cm (1 ft.) tall and weighing 250-480 g (0.5-1 lb.). No apparent sexual difference in size, although females generally heavier than males. Wingspan of 90-98 cm (3 ft.).
Colour/Appearance: Unmistakable white, heart-shaped face with tan/buff back and wing tops; pure white or speckled underneath.
Distribution: Fairly wide distribution in UK, although has suffered significant declines during the past 50 years. Absent from much of central and southern Scotland as well as northern-most Scotland and Scottish Islands. Absent from north-west Ireland. RSPB estimate UK wintering population to be 12,500 to 25,000 birds, with 3,000 to 5,000 breeding pairs.
Longevity: BTO ringing data suggest that most birds die at within a year of fledging, although the old wild-rung bird of which I'm aware from Britain was just over 13 years old while one from Switzerland lived just over 21 years. The Birds of North America gives the maximum longevity as 34 years old, although this is questionable.
Sexing: Difficult as there's no significant difference in size. The vast majority (~98%) of males exhibit a pure white breast, while in females the chest is flecked with black dots; juvenile females more heavily flecked than adults.
Activity: Typically hunt nocturnally -- best time to see them is dawn/dusk -- but may be active during the daytime, especially when feeding young and after nights of poor weather (i.e., wind and rain prevent hunting). They're often found sitting in trees during the day.
Territory/Habitat: Most often a bird of open country, hunting along field edges and "quartering" larger fields (i.e., flying low and covering an area in sections). Barn owls exhibit a preference for grassy headlands and fields with un-tended edges and hedges (with fence posts for perching). Also found along riverbanks and may hunt along roadside verges, where they're susceptible to traffic. Sometimes reported near urban areas. Generally resident, but birds will migrate substantial distances (500 km / 340 mi.) if food becomes scarce. Prefers secluded areas (e.g., barns, abandoned buildings, ruins etc.), especially for nest sites. May be found in cliff holes, mines, and holes in trees, but also take readily to nest boxes. It's arguably impossible to estimate territory accurately, but some suggest 4-5 sq-km (2 sq-mi.) is required to support a pair during winter.
Diet: Field observations and pellet studies suggest short-tailed voles are the most significant prey, along with wood mice and brown rats; ca. 90% of the diet is rodents. May sometimes take shrews. Occasionally take smaller birds, up to blackbird sized and they've been recorded hunting roosting starlings at Leighton Moss reserve in Lancashire and Aberystwyth Pier in Wales. There's a report from Sussex during 1985 of an owl killed by a dog as it tried to attack one of its puppies, but this was presumably aberrant behaviour rather than an attempt at predation. Food is swallowed whole and indigestible parts are regurgitated as pellet.
Reproduction: Typically monogamous, barn owls usually (but not always) form a life-pair. Productivity is linked to small mammal (prey) populations. Breeding condition is reached at 350-425 g (12-15 oz.) and the breeding season runs from March until August, with older birds breeding earlier. Copulation occurs after food presentation and they may also engage in mutual preening to strengthen the pair-bond. Egg laying date is affected by habitat quality (notably food availability); they start late in March/early April in good vole habitat (e.g., young conifer stands), while it's more likely during May/June in arable farmland -- most eggs are laid during April/May. Four to seven white eggs (up to 15 in good vole years) laid over several days, with about two days between each egg being laid. The female incubates alone, fed by the male who brings in an average of 9 items per day during incubation, increasing once the young have hatched. Eggs are incubated for around 30 days and hatch at two-day intervals, resulting in different aged birds in nest at same time. Eyes open at 8-11 days and skeletal growth is complete (i.e., the chicks can walk) by 5 or 6 weeks old. Wing growth is complete and the birds fledged at 8-10 weeks old. Ringing data suggest a very high mortality (up to 75%) during the first year.
Behaviour and Sociality: Lives singly or in pairs and hunts alone. Produces various different call types; a long, harsh chrrrrreeh call made while perched, and a long-drawn "rushing" sound during aggressive encounters.
Threats: Significant population declines across much of Europe and North America associated with numerous factors, including changes to farming practices. Susceptible to traffic while hunting road verges and agricultural rodenticides. Akin to all owls, they can be hit hard by prolonged bad weather (especially heavy rain and snow). They may also fall prey to peregrines and goshawks, and suffer kleptoparasitism (food-stealing) from kestrels and corvids.
Bibliography
Barn Owls in Britain: Phantoms of the farmyard - by Jeff Martin
Whittet Books
-- 2008
-- ISBN: 978-1873580752
Fauna Britannica - by Stefan Buczacki
Hamlyn Publishing
-- 2002
-- ISBN: 978-0600598671
Nick Baker’s British Wildlife: A month by month guide - by Nick Baker
New Holland Publishers
-- 2003
-- ISBN: 978-1845171131
Silent Fields: The long decline of a nation’s wildlife - by Roger Lovegrove
Oxford Univesrity Press
-- 2007
-- ISBN: 978-0199548156
Simon King’s Wildguide - by Simon King
BBC Books
-- 1994
-- ISBN: 978-0563364962
The Barn Owl - by Mike Read & Jake Allsop
Blandford Books
-- 1995
-- ISBN: 978-0713723496
The Book of the Barn Owl - by Sally Coulthard
Apollo Books
-- 2022
-- ISBN: 978-1789544770
The Darkness is Light Enough: The field journal of a night naturalist - by Chris Ferris
Sphere Books Ltd.
-- 1986
-- ISBN: 978-0718126902
The New Amateur Naturalist - by Nick Baker
HarperCollins Publishers
-- 2004
-- ISBN: 978-0007157310
Wild Animals of Britain & Europe - by Helga Hofmann
HarperCollins Publishers
-- 1995
-- ISBN: 978-0007627271