Plumage abnormalities

From time to time you may encounter a bird with plumage that is rather different from that which would be typical for the species. Some of these abnormalities may result from abnormal feather growth or feather loss, while others may be a consequence of problems with pigmentation.

Leucistic Blackbird by Dennis Simpkin

Blackbird by Dennis Simpkin

Changes in the amount and/or distribution of particular feather pigments are some of the most commonly reported types of plumage abnormality. Most often, these abnormalities occur within the common feather pigments, like melanin, and so we tend to see a pattern to the types of abnormalities being reported. Through our Abnormal Plumage Survey, we know that the most frequently spotted plumage irregularities are when birds gain odd white feathers. Interpreting exactly what condition these birds have can be tricky, as this article in Bird Table magazine explains.

Having plumage that differs from the population 'norm' is often disadvantageous, giving the wrong signals to other individuals or making the bird more obvious to would-be predators. Feathers with reduced pigment are often less robust and wear more rapidly, reducing flight efficiency and decreasing their insulative properties.

There are several different forms of plumage abnormality centred on altered amounts of pigment. These include leucism and albinism (where there is loss of the pigment melanin), melanism (in which the amount and/or distribution of dark-coloured melanin pigment is often elevated), erythrism (where a chestnut-red pigment replaces certain other pigments) and flavism (where there is an excess of yellow pigment). Both erythrism and flavism are thought to be rare compared with leucism and melanism.

If you have seen a bird with unusual plumage in your garden please complete our simple online questionnaire.

Helpful external links:
- Not every white bird is an albino: sense and nonsense about colour aberrations in birds.
- What colour is that bird? The causes and recognition of common colour aberrations in birds.