I thought I would share this, received from CJ WildBird Foods Newsletter and used with their permission:
'Every so often we encounter a bird that stands out from the norm. This individual is melanistic, meaning that it has an increased amount of dark pigmentation, in this case primarily on the head. We've only shown you the affected bit in this shot to make it a worthy challenge. The bill shape should tell you that it's a finch and, as it looks quite long and thin for a finch bill, may even narrow things down further. The bird is a common British garden visitor in all but the north of Scotland.'
The photographer was Pete Deans. CJ Wildbird Foods are found at http://www.birdfood.co.uk/. Excellent value for money, and they sell nest boxes, native trees, hedgehog and badger food, and loads more fantastic stuff, as well as first class wild bird food.