There are many different types of beak deformities, affecting either or both mandibles, in a manner of different ways. This Great Tit was first spotted here on the 3rd September, with what seemed at first to be just something stuck to his beak. In the time he was around on that day, it couldn't seem to shake it off (though it did seem to try). The (presumably) same bird was back again earlier this week, with the same object stuck, again trying to shake it off. This leads me to think it's not a beak deformity as such, though I'm not certain we'll ever be sure either way.
|Blue Tit with decurved (and slightly crossed) upper mandible|
|Rook with decurved upper mandible|
|Blue Tit with elongated mandibles|
|Goldfinch with leucism|
|Chaffinch with leucism|