Which type - foil-lined card, or plastic bottles (I would have thought the former, as translucent materials are presumably no good?).
I'm surprised that they pass the 'peck' test that the males are supposed to conduct at the entrance, to ensure that the walls are good and solid. Have you really seen this work?
If so, I'll try it with juice cartons. By the time I make nest boxes this year, it'll be too late.
