About the Barn Owl
Optimum habitat in Britain
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By far the best habitat for Barn Owls in Britain is rough tussocky grassland containing a high density of field voles - the Barn Owl's main prey. However, it would be wrong to give the impression that Barn Owls only use rough grassland and that they are absent from all areas that lack this wonderful habitat.
Virtually any area where small mammals can be caught by Barn Owls may be used for foraging. This can include the fringes of urban areas, as well as farmland and more natural habitats. Although Barn Owls are not woodland birds they will use woodland edges, rides, and large clearings - especially those with long grass. Barn Owls will also hunt around and within buildings, particularly those that are infested with mice or rats though this often brings them into contact with rodenticide poisons. In Britain Barn Owls are generally considered to be lowland birds but this is not because they avoid high altitude. Rather it is because uplands are usually dominated by intensive sheep grazing and/or lack sufficient rough grassland.
ROUGH GRAZING IS NOT THE SAME AS ROUGH GRASSLAND! This popular misconception is very important to clarify. In rough grazing, although the land may appear very rough (due perhaps to the presence of bramble, bracken, rushes, or scrub) the grass itself is usually very short (due to grazing by sheep, deer, rabbits, or cattle). Although these areas are often better for Barn Owls than intensively grazed "improved" or "semi-improved" grassland or arable monocultures, they are not nearly as good as rough grassland.
In rough grassland the grass itself is the dominant vegetation and (unless it's been agriculturally "improved") consists of a wide variety of native grass and herb species. The most crucial feature of good rough grassland is the presence of a "litter layer" at the base of the grass.
Grass that's allowed to grow tall (knee high or taller) in the summer, and is not removed, will collapse in the autumn and by the following spring fresh green blades will grow above the horizontal layer. By the following summer most of the first-year's growth will have died and formed a "litter layer" or "thatch" about 75mm (3") deep. Through this layer the field voles will already have started to make their "runs" - a matrix of small tunnels leading to food stores, latrines, and nests - and in the winter you'll be able to spot small holes (about 40mm in diameter) through which the voles emerge to graze (they mainly eat grass).
Field voles do occur in other types of long grass (such as hay meadows that are short-mown annually) but without a permanent litter layer their numbers will remain relatively low. In farmland areas there may be as few as 15 field voles per hectare (1 hectare = 2.5 acres) whereas in rough grassland field vole density can range from 25-250 per hectare. During a peak year field vole density may reach 400 per hectare in perfect rough grassland (for further information see Harris & Yalden, 2008, Mammals of the British Isles Handbook).
There is more information about this in the Barn Owl Conservation Handbook.