Leucism vs albinism - a pedant's guide!
I was doing a bit of reading last night and found that the term "partial albino" has gone into print. I thought I would look up the precise definitions to see if the term has any meaning. There is no precise definition for leucism but it is the difference in the two traits that is interesting. Albinism is a complete lack of the cells that produce pigments whereas these cells are 'switched off' or faulty in the leucistic state. Now the difficulty comes in deciding that abnormal white patches on a a bird are caused by the lack of pigment producing cells in those areas or because they are faulty. In some cases, white or patchy birds moult into normal adult plumage and this infers that leucism is the state. However, the local may have a single wing feather that is always white and this would infer that there were no cells to rpoduce pigments on a localised basis. Given that the feather never attained normal colouration, it would probably be true to refer to the bird as an albino even though it was normal in all other respects. However, the bird that I use in my avatar (a meadow pipit, we think) has no feather pigmentation yet there is colouration in the bill, eyes and legs. It is difficult to decide if the pigment cells are switched off or absent in this case so again, it is probably OK to refer to this bird as an albino. Nevertheless, many people do not consider a bird to be albino unless it completely lacks pigmentation. In conclusion, the term "partial albino is mostly useful because it is almost impossible to determine which state is the actual cause without detailed examination of the bird. I am not sure this helps really but I found that it is not as straightforward as I first thought.
'The Truth we learn by turning stones' - Judie Tzuke