I wonder about this migration mallarkey. I read of a blackbird which lived in Norfolk in the summer but migrated to Devon for the winter. Sounds like a bloody good idea.
These Brighton swifts in early October - maybe they are in transit from somewhere else? Given that the species gets up to the Arctic Circle, maybe they were heading south from Norway?
As I posted elsewhere, I saw house martins on the Westcliff sea front on 20th November 1976. Never seen them, or swallows, later than 3rd week in October in any other year. We've got southerly winds blowing atm. I wouldn't be surprised if they've just come back for some baggage they forgot!