Barn owl numbers have declined over much of the British Isles. However, in northern Britain afforestation has resulted in some local increases. Young plantations with rank grassy vegetation contain large numbers of field voles which are the main food of barn owls. Abandoned farm buildings provide nest sites for the owls, but these deteriorate and are unavailable for nesting by the time the forest is felled and foraging conditions are again suitable for bam owls. Even in afforested areas, the low density of buildings may limit the population of bam owls. This Bulletin presents a summary of the work in forests of south-west Scotland where surplus nest boxes were provided to see if the density of barn owls could be increased. During three years when field vole populations were increasing, the barn owl population in the nest boxes increased from 0 to 31 pairs, demonstrating that the bam owl population had previously been limited by a lack of nest sites. Barn owls were abundant on the farmland area adjacent to the forest, and chicks produced from these traditional sites colonised the forest sites. Recommendations are also given on how the results from this study may be applied in other forests.