Abnormal Plumages

Yesterday, as I was driving along Harbor Blvd. in West Sacramento, I saw a small group of House Sparrows coming down to the street. They were feeding on something they were finding along the gutter. By itself, this would not be a particularly interesting sight. House Sparrows are everywhere in West Sacramento as they are in practically every urban area in North America, and seeing a small group at this particular location is pretty normals as well. What drew my eye on this occasion was the fact that one of the birds had a white head! As I drove closer, I saw that its head was not completely white, but instead had a bit of a dark line over its eye. It also had a white patch on each wing and a bit of white in the tail (maybe one feather on each side). These white feathers were, as far as I could tell, pure white, and the other feathers on the bird had normal House Sparrow coloration. This got me thinking about plumage variations and the associated causes and terminology.

Bird pigmentation, and the terms we use to describe that pigmentation, can get a little complicated, but it goes something like this. Albinos are individuals that have a genetic mutation that prevents them from depositing any melanin anywhere in their body. This results in individuals that have red eyes, and are generally white on their bodies. However, albinism only effects melanin production, so birds that use other molecules as pigments, such as carotinoids, may still have coloration. They either look yellow-ish, orange-ish, red-ish, or have parts of their body that have yellow, orange, or red feathers while the rest are white. In birds, there are two types of melanin: eumelanin and phaeomelanin. Albinism occurs when both types are completely absent. When only eumelanin is present birds will lack the warmer chestnut and buff tones and so will appear plainer, paler, and greyer. When only phaeomelanin is present birds will lack the color black, and also have generally paler wing and tail tips. In birds that only black, such as crows, this results in a completely white bird. When both types of melanin are present but only at low levels, birds will appear to have pale, washed out versions of their usual plumage. This is referred to as a dilute plumage.

Leucism is a genetic mutation that affects all the pigment producing cells. Fully leucistic individuals are completely white, often confused with albinos, but they still have dark eyes (pigments cells in the eye are derived from a different line of cells than pigments cells in the rest of the body). Partially leucitsic individuals have certain patches that have been effected by this mutation, and so are white, and other parts of the body that have not been effected, and so are normally colored. This ‘pied’ look is what I saw in that House Sparrow, so that bird is partially leucistic.