The paper by Davis has an interesting set of illustrations showing the progressive loss of melanin from the plumage of a Stellar's Jay over the course of a single moult (see p. 39). In the last three years we've had a female house sparrow regularly visiting our garden in Whanganui East that seems to show similar progressive loss of melanin over successive moults. Although the bird is unbanded, I'm assuming that it is the same bird, based on the regularity of its occurrence (at least once a week) and that I've never seen two aberrant coloured females in the garden at the same time or even within a short time of each other. In fact, it appears to be the only aberrant coloured female in the vicinity. She reared two broods during the last breeding season (2018/19) and, on each occasion, her 2-3 chicks appeared to be normal coloured (she fed them for about a week post-fledging with grain put out for the birds).
Here is a set of photographs showing this progressive loss (assuming that it is the same bird). Her post-breeding moult is in late-February - March, during which time she replaces all of her body feathers.
- December 2017
- Leucistic female sparrow_Dec 2017_lowres.jpg (563.41 KiB) Viewed 2233 times
- May 2018
- Leucistic female house sparrow_May 2018.jpg (464.58 KiB) Viewed 2233 times
- April 2019
- Leucistic female house sparrow_LHS_April 2019.jpg (459.1 KiB) Viewed 2233 times
This progressive loss of melanin over time suggests either creeping old age or some environmental cause. I've no idea what the latter might be as the bird looks healthy and is lively (sprightly might be a better term). Interestingly, when flying, she seems to have a faster wing beat than ordinary sparrows but this could just be an optical illusion. Alternatively is could be that with much reduced melanin in her flight feathers, they do not have the same rigidity of normal feathers, and this may require faster wing beats to compensate. This draws attention to the question of what the consequences of plumage abnormalities could be, apart from the obvious one of (perhaps) increased vulnerability to predation.
Peter
Not_every_white_bird_is_an_albino_Sense_and_nonsen.pdf
