3 September 2013
Deformities have been recorded in a wide range of birds. They can be caused by injury, genetic defect and disease and most noticeably affect the beak and legs of birds.
Less than 1% of birds examined in the hand have had deformities, perhaps reflecting their low survival rate.
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.
A bird's beak consists of an upper and lower mandible, both of which have a bone base with a horny covering that continually grows, mainly at the tip, to make up for the wear and tear of use. Any change in the normal growth and wearing processes may lead to the formation of an abnormally shaped bill.
If the tip of a mandible is broken off, the opposing mandible will grow unchecked with upper mandibles elongating in a downward curve and lower mandibles curving upwards. Mandibles that have become displaced laterally, so that their tips are not opposing, often both grow abnormally forming a slightly crossed bill.
Inherited crossed bills have been recorded in some species but information is sparse as congenital defects are difficult to study. One common deformity, frequently seen in starlings, is thought to be congenital and appears as an elongation of both mandibles which are also curved downwards causing a 'curlew-type' bill.
Disease and parasites can affect the growth area of a bird's bill and both avian pox and trichomonosis are thought to have caused abnormalities in woodpigeons.
A bill deformity will often affect the behaviour of a bird. Birds whose bills become gradually deformed have time to adapt to their disability whereas those who suffer a sudden fracture may find it more difficult. Feeding can be a problem and many birds with elongated or broken bills will use the sides of their bill instead of the tip.
Birds with a slight deformity will be able to feed almost normally and so avoid excessive growth of their bill: those which have to change their feeding methods are more likely to suffer abnormal growth. The daily activity of feather maintenance may be impaired, the unpreened plumage harbouring an excessive number of feather lice. Other parasites may also be present in large numbers, affecting the health of an individual.
Crossbills, parrots and birds of prey, whose mandibles do not oppose, have no problems with increase in bill length, probably due to the natural wear from their feeding habits.