Albinism describes birds in which some or all of the normal pigmentation is missing. It is most often inherited, but can be caused by other factors.
As it is a recessive characteristic, it only shows up when a bird inherits the albino gene from both parents. There are different degrees of albinism, ranging from all white to only a few white feathers on an otherwise normal coloured bird.
True albinos lack colouring in all feathers and soft parts. Their feathers are white, and bill, legs and eyes are pink. True albinos are normally less robust in other ways and often have deficient eyesight and hearing. They rarely survive for long, and therefore are only rarely seen.
Most reported albinos have normal eye, bill and leg colouration. Being generally conspicuous, they are more likely to fall prey to predators. Some albinos are shunned by other birds and will find it difficult to obtain a mate, but this is not always so.
Partial albinism is caused by the failure of pigmentation to reach certain feathers. While this is often hereditary, other factors such as unbalanced diet, old age and injury, or even disease and shock can cause albinism. In hereditary cases the white pattern is consistent from one moult to another, but albinism caused by environmental factors is often reversible.
Sometimes red and orange pigmentation is retained while melanin is absent. Hence, some albino robins and bullfinches often have reddish breasts, and a green woodpecker retains the red crown. This condition is sometimes separated from albinism as schizochroism.
If a bird has a deficient diet during feather growth, white feathers can appear as melanin deposition into the growing feather becomes interrupted. This is well documented in crows and rooks. An initially deficient but improving diet may produce pale-tipped feathers, and brief or intermittent periods of starvation can produce one or more traverse bands across feathers.
It is thought that the white wing patches on young crows may be due to dietary deficiencies in the nest, since they normally disappear as the affected feathers are moulted.
Progressive albinism can occur as a bird gets older, in the same way as people go grey. This has been recorded particularly in blackbirds.
Partial albinism can create significant problems with identification. For instance, part albino chaffinches and house sparrows are often reported as snow buntings.
Over 160 species of British birds have been recorded showing partial or complete albinism. Only a minute proportion of wild birds are albinistic.
A survey showed that six families were the most susceptible to albinism and provided two-thirds of the records. 29% of recorded albinos were thrushes (especially blackbirds), while crows (especially rooks and jackdaws) accounted for 11%.
Swallows/martins (especially swallows), sparrows and starlings each made up 7% of the reports, while 6% of reports involved finches. Albinism is reported most often in dark coloured species that are commonly seen in urban areas. This may be because of the higher chances of these birds being seen.