A Bottle Blonde?

I have been watching this bird for the last few days, wondering what on earth I am seeing. It is a blackbird (or so I think) and has been visiting the apple as the blackbirds do, several times a day. But it has a white head or, what looks to be stubbly white quills on its head. A mutant blackbird I thought - until today when I saw a white headed bird at the apple again, but the beak was a dark colour. That means there are two mutant birds.

I lay in wait with the camera but it was the orange beaked one that came back. I have put a close up in the extras for those as curious as I am.

Any ideas as to why or what ?