In reply to Richard Ellis's comments about Chiffchaffs I believe he is right about the wintering birds departing before the migrants arrive. Every year we have a wintering Chiffchaff in and around the garden. Every year, they vanish about mid Feb, and then there is a Chiffchaff-less gap until the migrants arrive in mid-March. Our resident winter Chiffchaff (sometimes 2) never sing before departing. But then, they could be females. (Do we know what sex our wintering birds are? Often there is a sex difference in the location of wintering/dispersed birds)
This does not explain Richard's early singing bird in Carew though. Maybe there are always exceptions!
Rosemary Royle