Colour aberrations in birds are not uncommon and many observations have been published over the years in the ornithological literature. Unfortunately, ornithol- ogists tend to use descriptive names that refer to the colour of a particular bird with unusual plumage. However this is confusing, as for each species the normal plumage colour varies, and therefore the appearance of any given mutation will be different for each species.

Albino and Leucistic are the two most misused names for plumage aberration. Alba and Leukos both mean white, but both names are often mistakenly used for colour aberrations that involve no white feathers at all. Brown is the most common colour mutation in birds (van Grouw 2006, 2010) but in records of observations of this particular mutation it is called by many different names. Albino, Fawn, Isabella, Leucistic, Cinnamon and Pale morph are the most common terms but the same terms are also often used for Dilution. Mutations should not be named in relation to the appearance of the final plumage colour, as this can differ between species. Rather, they should be named in relation to the relevant gene action on the pigmentation process as this is the same each time.

The most common pigments in birds that give the plumage its colour are melanins. Melanins can be distinguished in two forms: eumelanin and phaeomelanin. Depending on their concentration and distribution within the feather, eumelanin is responsible for black, grey and/or dark brown feathers, whereas phaeomelanin is responsible for warm reddish-brown to pale buff. Both melanins together can give a wide range of greyish-brown colours. In many species the (adult) colour is caused by eumelanin only, as is the case in the Common Guillemot Uria aalge.

The development of the actual melanin that is deposited and distributed into the feathers is the result of a chemical process within special cells called the melanin synthesis. The final colour stage of eumelanin is normally black but due to the gene for the mutation Brown the melanin synthesis is incomplete which results in brown eumelanin. A Carrion Crow Corvus corone with the mutation Brown will appear chocolate-brown. However if the normal colour of a certain species is the result of dark brown eumelanin instead of black, as in the Common Guillemot, the mutation Brown will cause a light brown plumage colour in that species. So while the final appearance of Brown may be different in different species, the gene action is the same in all species: an incomplete synthesis (oxidation) of eumelanin. Brown can therefore be defined as a qualitative reduction of eumelanin. In this mutation the amount of pigment remains unchanged but the appearance of the eumelanin is changed.

This paper evolved from Short Notes submitted independently by SR and OM. We thank Rob Barrett, Mike Harris, Bryan Nelson, Sarah Wanless and Andy Webb for their helpful comments, Steve Duffield, Hugh Harrop, Martin Heubeck, Neil Lunn and Martin Scott for help in sourcing photographs, and the photographers concerned for allowing their use.

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