The Barn Owl is one of the most widespread of all owls and, indeed, is among the most widely distributed of all land birds. Versatility in the use of nest sites and in selection of prey, strong powers of flight, and an ability to use human-modified habitats undoubtedly are significant factors in the large geographic range of this species. Despite being common in some areas and often nesting close to human habitations, the secretive, nocturnal activity of Barn Owls renders them inconspicuous to most people. Declining populations in several areas have raised public awareness of the species. The Barn Owl is one of the most intensively studied owls, especially in Europe and North America, but most of the 28 subspecies remain poorly known ( Bruce 1999 ).
This owl occupies a broad range of open habitats, urban to rural, favoring lower elevations in most of its range, although it occurs to >4000 m in the Andes of South America. It extends much farther north and south than any other member of the family Tytonidae, most of which are tropical to subtropical in distribution. Its northern range limit is determined by climate, specifically the severity of winter conditions. Barn Owls nest in a wide variety of cavities, natural and those made by humans: trees, cliffs, caves, riverbanks, church steeples, barn lofts, haystacks, and nest boxes. Its breeding numbers seem limited by the availability of nest cavities in proximity to adequate densities of small mammals (especially voles [Microtus spp.]), its primary prey. Its reproductive pattern is highly flexible, especially compared to other owls. Generally monogamous, it is sometimes polygamous and can raise two or more broods per year. It can breed year round where climate permits. Normally a strictly nocturnal species, the Barn Owl has evolved excellent low-light vision and remarkable hearing; indeed, its ability to locate prey by sound is the most accurate of any animal tested. Changing agricultural practices threaten some populations, but nest boxes have helped to boost numbers in other areas.
Life history and distribution of this species have been summarized by Cramp ( Cramp 1985a ) and Snow and Perrins ( Snow and Perrins 1998a , both for the Western Palearctic); Marchant et al. ( Marchant et al. 1999 ; for Australia and New Zealand); Ali and Ripley ( Ali and Ripley 1969 ; for India and Pakistan); and Bruce ( Bruce 1999 ; worldwide). Interested readers should consult these sources for detail specific to those regions. Bruce ( Bruce 1999 ) provides a particularly thorough and well-illustrated treatment.