The RSPB reports that the number of multiple occupancy nest boxes is on the rise. I never knew it but apparently kestrels have been seen to shack up alongside barn owls amongst other strange flatmates, and the problem is getting worse over the years. This may be because of a general decrease in suitable nesting spots, so why not put up a nest box in your garden? Birds often roost in them overnight during cold periods, so it's worth doing so now rather than waiting till next year. Put some sawdust shavings in the bottom to make it homely.