I checked a double pole box tonight. The owners suspected Tawny Owls might be nesting in the top box. I had a quick look in the bottom box first and found a single Tawny Owl chick. Before ringing it, I thought I'd quickly check the top box and as I got closer I could hear bill snapping. I wondered if mum was roosting in it, but when I opened the door I found Barn Owl incubating! It will be interesting to see if both species successfully fledge young, particularly as I have not found any cached rodents yet and the Tawnys seem to be surviving on birds.