We have a scheme running in Herefordshire where you buy a barn owl, little owl or kestrel box (made in a prison workshop) and it's inspected twice a year - squirrels and jackdaws are evicted. They're coming to inspect ours today. Apprently it's often taken up by youngsters that use it for shelter over the Winter, so can take some time to be used. Everything's early this year and they've already found one box newly occupied mid-County.