Causes of change

The use of toxic farm chemicals, loss of hunting habitat, increased disturbance, hard winters and the increase in traffic collisions have all been suggested as possible reasons for decline, but clear evidence is lacking. The upturn over recent decades has been aided by conservation measures including the widespread erection of nestboxes.

Further information on causes of change

Decline during the 1950s and 1960s was probably associated with use of toxic farm chemicals (especially organochlorine seed dressings), but also loss of hunting habitat, increased disturbance and the hard winters of 1946/47 and 1962/63 (Dobinson & Richards 1964, Percival 1990).

Causes of mortality potentially linked to the species' further decline include poisoning (Shawyer 1985) and collision with road traffic (Bourquin 1983, Massemin & Zorn 1998, Shawyer & Dixon 1999). Barn Owls are vulnerable to secondary poisoning from ingesting rodents killed by 'second-generation' rodenticides, which are used to control warfarin-resistant brown rats Rattus norvegicus (Shawyer 1985, 1987, Harrison 1990). Toxicological studies found that a small proportion of dead Barn Owls contained potentially lethal doses of rodenticide (Newton et al. 1991; Newton & Wyllie 1992a). There is no clear evidence, however, that links either poisoning or traffic collisions to population changes.

More recently, the erection of Barn Owl nestboxes, already numbering c. 25,000 by the mid 1990s, may have enabled the species to occupy areas (notably the Fens) that were previously devoid of nesting sites, and may have been a factor in improving nesting success (Dadam et al. 2011). In earlier decades, the plight of such a charismatic and popular bird led to extensive releasing of captive-bred birds in unguided attempts at restocking: by 1992, when licensing became a requirement for such schemes, it was estimated that between 2,000 and 3,000 birds were being released annually by about 600 operators, although many birds died quickly and never joined the nesting population (Balmer et al. 2000). There is some evidence, however, that releases might have aided population recovery (Meek et al. 2003).

The Barn Owl is a specialist predator of small mammals, in particular voles, mice, shrews and small rats (Shawyer 1998), but frogs and small birds are also taken (Bunn et al. 1982). The field vole Microtus agrestis, the most important prey of Barn Owls in mainland Britain (Glue 1974), favours grassy cover and a thick litter layer (Hansson 1977). In the UK, positive relationships were found between abundance of small mammals and sward height (Askew et al. 2007), whilst other authors have found a positive correlation between bank voles Clethrionomys glareolus and the width of grassy field margins (Shore et al. 2005). In Switzerland a similar result was found between unmown wildflower and herbaceous strips and densities of small mammals Aschwanden et al. (2007). Foraging of Barn Owl in an arable landscape is largely restricted to uncultivated or ungrazed field margins (Andries et al. 1994, Tome & Valkama 2001). It has been estimated that Barn Owls breeding in arable landscapes need about 35 km of rough grass margins within 2 km of the nest for the population to remain stable (Askew 2006).

Variation in adult survival contributes most to annual population changes (Robinson et al. 2014). Barn Owls experience reduced hunting opportunities in snowy or wet weather (Shawyer 1987). The recent downturn, after two decades of positive trend, may have resulted from a series of cold winters, during which higher-than-average numbers of individuals were reported dead (Clark 2011, Demog Blog). Poor hunting conditions in spring and summer may decrease adult or chick survival or reduce adult body condition, with associated lower investment in reproduction or, in some cases, the suspension of breeding (Shawyer 1987). Vegetation growth may also be affected by cold weather, with implications for the abundance or availability of small mammal prey (Shawyer 1987, Clark 2011).

Information about conservation actions

The ecological requirements for this species are reasonably well understood and therefore a number of conservation actions have been suggested, although the most important drivers of change are still uncertain.

Barn Owls take readily to nest boxes, and the increases in recent years are likely to have been aided by the widespread provision of boxes. Nest boxes can be a useful conservation tool in areas that are devoid of nesting sites (Dadam et al. 2011; Petty et al. 1994); however Klein et al. (2007) found that pairs nesting in boxes had lower productivity rates and recommended that partial reopening of buildings should be preferred if suitable buildings were available.

Provision of suitable habitat is also important and conservation actions and agri-environment policies that enable the provision of foraging habitat may also benefit Barn Owls. It has been estimated that Barn Owls breeding in arable landscapes need about 35 km of rough grass margins, 4-6 m wide, within 2 km of the nest sites for the population to remain stable (Askew 2006). Areas of rough grassland cut every 2-3 years supported more Barn Owl prey than areas cut annually though this was only significant for common shrews Sorex araneus (Askew et al. 2007). In Sussex, land cover was less heterogeneous at successful breeding sites, with home ranges characterised by a few habitat types of regular patch shapes; unsuccessful sites had significantly more improved grassland, suburban land and wetlands surrounding them (Bond et al. 2005).

Anticoagulant rodenticides (rat poisons) have been found in some Barn Owl carcasses in the UK, in most (but not all) cases at low concentrations which were unlikely to be considered a contributory cause of death (Walker et al. 2013, Walker et al. 2014). Nevertheless, precautions such as prompt removal and safe disposal of poisoned rates, as well as continued monitoring, would be prudent.