White swallow?

I was contacted by my farming neighbour Oliver John, who tells me he had what appeared to be a white swallow hawking over his fields as he brought his herd in for milking yesterday. Oliver is no expert but must have seen a few squillion swallows over the years so I am happy he is right.
I remember seeing a stuffed one in a case some years ago. I will be going with him to see if its still around, today, allthingsgood, walrus!