Chris Payne trapped this Great Tit with a strikingly malformed bill in Greens Norton recently. It was trapped twice and appears to be coping well with its deformity and maintaining its average weight.
Such deformities have occurred in a number of passerine species and on Flickr, there is a page devoted to photos of Birds with Deformed Bills. Several possible reasons for such
deformities have been put forward, including genetic or developmental causes, injury or disease, although agricultural chemicals and pollutants may also play a part. It is believed that while some birds can adjust to their handicap and may feed by turning their heads to one side others apparently starve or are plagued by numerous parasites, such as body lice according to Passerines with deformed bills (Julie Craves).
Northants birds 