Here, they are definitely migratory--tho I have seen them fairly late in the fall. Sadly, I have known them to get caught here also, having removed four mature dead ones from a single box one spring. I never realized what Squid observed about bb's and ts's. I have boxes on two utility poles about 10ft apart. One always has bb's and the other has tree swallows.
I don't have any troubles w/wrens tho. Are we talking house wrens or Carolinas?? My Carolinas always nest in my wood piles while my house wrens occupy a much smaller house of their own w/a one inch hole.
I confess I've failed miserably at attracting a colony of purple martins. They check me out yearly but never stay. I think hosps are the problem tho I work hard to keep them at bay.
