Why such a strange beak?

May I have your observations on the attached photo taken in our rear garden. I have seen images with extended upper beaks but nowhere near as long as this one. The bird seemed to be feeding well. How and why does this happen, please? Jack Hansley, Saxilby, Lincoln

QAbnormalities in wild birds are quite rare, but when they do occur it is usually the feet or beak that are affected. Lower bill deformities are less common than upper bill deformities but do occur, and there are various causes. Some are caused by deficiencies in diet, or by chemical poisoning, but the commonest cause seems to be damage to the beak. In the case of your Rook, we think that the bird has at some point broken off the tip of its upper beak. As birds’ beaks grow fairly continuously, this will have caused the lower mandible to grow past the upper, and begin to curve upwards, while the upper beak grows back. As long as the bird suffers no more damage and can feed normally, there is no reason why it won’t survive quite well, although it may have a few problems grooming itself.

A