Back to OT (I take it that a "band" is human and so off-topic)...
Mrs N & I were visiting Cornwall a month ago and saw several swallows and house martins. I mentioned it to one of the caravan site wardens, who said she had seen house martin nests with youngsters in Newlyn last December. There are lots of insects around, even in the frozen north of Warwickshire (saw a dragonfly today inter alia),
I speculate that they might have enough food, in a good year, to give up migration.