Every now and then I come across a bird sporting some white feathers. Perhaps it is because these white feathers stand out most on a bird that is otherwise black, that the species involved is often a Blackbird, Jackdaw or Carrion Crow. Such plumage abnormalities are, however, likely to be more widespread than this and this is why the BTO is carrying out a study to examine the incidence of abnormal plumage across a range of species.
The presence of one or more white feathers often results from some non-heritable cause, such as disease, food deficiency or trauma. In other cases, the cause may be under genetic control. Perhaps the best-known of these is albinism or partial albinism. However, the definition of what constitutes an albino or partial albino is rather precise and most of the cases involving aberrant white feathering in birds are usually either a form of leucism or result from some non-heritable cause.
White feathers can occur where the normal colour pigments (e.g. melanin) are missing. The resulting feathers are often weaker because of this and, therefore, more prone to wear and damage.
In some instances, the process works the other way and too much melanin is produced, resulting in feathers that are far darker than usual. There is, for example, a population of Great Tits in southeast England that have completely black heads, the white cheek feathers turned black by abnormal levels of pigment.
Other aberrations can produce feathers that appear washed out or, alternatively, more strongly coloured with a particular pigment. In some cases these may result from diet; I can recall a case from a few years ago where House Sparrows that had been feeding on fish pellets at a Salmon farm in Scotland had feathers that were a muddy shade of pink! The pellets contained a natural pink dye, used to give the Salmon a stronger colour, favoured by customers.
One of the main problems with understanding the different abnormalities is that the naming of aberrations is based on the relevant gene action on the pigmentation process and not on the appearance of the final plumage colour. As observers, we see the final result not the process that brings it about, making it difficult to interpret which abnormalities occur in the wild. The BTO survey will provide some useful background figures but it will require detailed work to more fully understand the processes operating on individual birds. If you have seen a bird with abnormal plumage then the BTO would like to hear from you. You can either complete the survey online (www.bto.org/gbw) or request a paper recording form from, BTO Plumage Survey, The Nunnery, Thetford, Norfolk, IP24 2PU. Alternatively, email firstname.lastname@example.org.