No, they in no way impede migration. And in the temperate parts of North America, at least, where most of the local cavity roosters are resident year round, I doubt if nest boxes see any significant use by migrants.
Bird photos (Flickr): http://www.flickr.com/photos/fugl/
". . .Let them be left, O let them be left, wildness and wet;
Long live the weeds and the wilderness yet."
--Gerard Manley Hopkins
Last edited by fugl : Wednesday 30th October 2019 at 06:24.