Of course the winter atlas started on 1 November, so some of these could be late stragglers, rather than truly overwintering birds.
Of course, that's a good point.
However, I've searched the local news achieve on RBA, which lists two Whinchat reports since 1st November 2007:
- A bird from 1-3rd Nov at Swallow Moss, Staffs
- An unconfirmed report of a bird at Minsmere, Suffolk on 30th Nov.
And that's it. Note the complete lack of overlap with those shown by the BTO to demonstrate the wintering population of Whinchat's in the UK.
From this one could assume that the birds logged with Birdtrack were not reported by local birders through other means as monitored by the various bird information services. I know when a bird tried to winter in Bucks a few years back it was frequently reported. This strikes me as strange.
As Poecile says, slightly tongue in cheek, this is a concern for the atlas and the accurate monitoring of birds in the UK. That, to me, is why ID matters.