Beak deformities in wild birds are unusual, with fewer than one in 200 adult birds thought to be affected. However, a number of reports from BTO Garden BirdWatchers have caught our eye, and we are keen to find out more about the deformities being seen in British and Irish gardens.
Deformities have been reported in over 60 species of birds worldwide, from sparrows to pigeons, warblers to gulls. Although some are temporary – and perhaps controllable by deliberate abrasion – others are permanent, diverse and often spectacular in their form.
Help us to understand the causes of beak deformities
By collating reports of beak deformities, the species involved and the nature of the abnormality, we hope to identify common patterns. By submitting your sighting to our simple survey you can help us to build up a better picture.
Causes of beak deformities
A number of probable reasons for beak deformities have been suggested, but in many cases exact causes are unknown.
Exposure to environmental contaminants, such as industrial pollutants, insecticides, herbicides and fungicides, have been implicated but evidence to support this more widely is not well established.
Nutritional deficiencies could lead to beak abnormalities. In isolation, some human-provided foods are unlikely to provide a complete diet and can differ in texture compared with natural alternatives, perhaps causing differences in patterns of beak wear. For example, in some captive raptors where the tips of the upper and lower mandibles do not oppose each other, beaks can overgrow if individuals are not provided bones at which to pick.
Physical trauma (e.g. due to a collision) to the beak is a possibility is not a major factor.
Beak deformities could be influenced by disease or may be inherited genetically. While research continues to investigate these possibilities, current evidence is not comprehensive.
Types of beak deformity
The different types of beak deformities can be grouped into several distinct forms, reflecting their basic appearance. These are:
Crossed mandibles: similar to that of a Common Crossbill and perhaps caused by slight asymmetry of the jaw. Straight beaks have been recorded as a deformity in Crossbills.
Decurved upper mandible: can occur when the tip of the lower mandible is damaged so that the beak does not close correctly. Similarly, upcurved lower mandible may occur when the tip of the upper mandible is missing (i.e. broken off).
Elongation of both mandibles: produces ‘Curlew-type’ beaks, although the degree of curvature varies considerably between individuals.
Bent to the side: seemingly uncommon, with one or other mandible warping sideways.
Gapped: where the upper and lower mandibles do not close fully leaving a visible gap.
In most cases the bony parts of the beak are unaffected, with abnormalities occurring in the dermotheca (the thin middle layer containing blood vessels and nerves) and rhamphotheca (the outer keratinous sheath). Researchers into beak abnormalities in Alaska have coined the term ‘avian keratin disorder' and have noted additional abnormalities of feathers, skin, legs, feet and claws in some wonky-beaked individuals.
Consequences
Many birds adapt to their unusual beak and a surprising number remain in good condition, even able to find mates and breed. Successful adaptation appears to be more likely if the change in beak morphology (i.e. its form) occurs gradually rather than as a result of sudden injury.
Some birds can compensate for their deformed beak by feeding with their heads tilted to one side. Interestingly, some can even apparently benefit from their ailment. For example, one deformed Great Tit was quicker at extracting nuts from their shells than a normal individual. Drinking may also present a problem and a Blackbird with an abnormal beak has been observed collecting valuable drops from beneath leaves or drainpipes. Birds with damaged beaks are often drawn to garden feeding stations or other human-provided sources of food.
Birds with beak deformities can have difficulties preening, leading to an increased number of parasites in their plumage which, in turn, can make them less attractive to a prospective mate. Plumage that is in poor condition also functions less well, increasing the risk of mass loss and hypothermia.
Research in Alaska shows that some nesting female Black-capped Chickadees (similar in size and appearance to our Coal Tit) with deformed beaks act erratically, scattering eggs about the nest cavity rather than positioning them neatly in the nest cup. This might be because a deformed beak is cumbersome when turning eggs (an essential part of successful incubation, or because of hormonal abnormalities altering her incubation behaviour). Perhaps unsurprisingly, deformed females tend to hatch fewer eggs and deformed males struggle to rear their young.
In many species, males feed their mates during the breeding season, but when a male’s beak is deformed this process can become unwieldy. A smart female Great Tit learnt to circumvent this problem by tilting her head to one side so that she could accept nuptial gifts from her wonky-billed mate. Interestingly, problems for males do not stop there. Deformed male Black-capped Chickadees have been found tending to nests with a higher proportion of eggs that are not their own compared with the nests of normal-billed males - females seeming to covet a bit on the side with a more 'healthy' mate.