With this ability, the Barn Owl can hunt effectively both in the absence of moonlight and when the prey is completely hidden by low vegetation or leaf litter. Barn Owls nest in natural cavities such as trees or cliffs as well as in man-made structures such as church steeples, barn lofts, attics, and nest boxes. Both the numbers of available nest sites as well as the availability of prey animals appear to limit the number of breeding pairs in any given area. In areas such as the midwestern U.S., declining Barn Owl populations caused special concern in the early 1980s. Six states declared the species endangered and nine others, worthy of special concern.
Populations seem to be in decline in Europe also.
Chemical residues have been identified in wild Barn Owls as well as in their eggs, and their eggs have been found to have undergone significant eggshell thinning. Laboratory tests have found that pesticides will cause death and eggshell thinning. Intensive farming practices involving the spraying of pesticides on crops are thus thought to have had a negative effect on the bird's populations. In the northern parts of the species' range, many Barn Owls die as a result of extreme winter weather, either because of cold temperatures or because of inadequate prey availability. Another major cause of death among the owls is collisions with cars.