Deformities of the feet are caused mainly by disease and injury and are most noticeable among ground feeding birds, especially in towns and cities.
Cuts to the feet can allow infection to occur which may result in swelling and lameness.
Bumblefoot, which affects large birds such as crows and birds of prey, is caused by the entry of bacteria through a break in the skin, producing inflammation which results in difficulties with perching and walking.
Avian pox can cause wart-like growths on the feet of house sparrows, starlings and pigeons. Another viral disease causes tumours of the feet, particularly in chaffinches.
A common disability among urban birds is the loss of all or part of a foot due to a discarded thread or wire. Discarded fishing line can have a similar effect in the countryside. Many birds can survive an amputation but any resulting infection will usually prove fatal. Congenital deformities involving extra or missing toes are rare and birds with gross abnormalities, such as an extra leg, do not survive.