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.

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.

Click here to read the rest of the article.: Plumage abnormalities | BTO – British Trust for Ornithology