We have had 3 or 4 pairs together, nesting in our roof. Corrugated metal with bird-proofing. The bird-proofing came in a long strip to fit the profile at the open edge of the roof. Way back when, I see one of the labourers with a pair of scissors. Carefully cutting that strip into single-slice-of-bread-pieces. What is he DOING?! Ah well, sheepish look, we forgot to put it on, and now the roof sheets are screwed down. As the years pass, winter wind and delighted birds are ripping those bits out, one by one.