These certainly are cases where the unfit survived. Natural selection has not operated. But such cases are rare exceptions.
DUST AND SNOW BATHING
The taxidermist preparing a bird specimen for the museum sometimes has to deal with one whose plumage is soiled or stained. He may have to wash it with water. Then, to dry the plumage, fluff it, and help in arranging the plumage so it will lie smooth and natural, he may use a powder: corn meal, sawdust, plaster, or plaster and potato starch may be worked into the feathers, then dusted out again. It is interesting that birds themselves use and have used long before taxidermists a similar method of using dust in dressing their feathers, a fact that anyone who has watched domestic hens for any length of time must be aware.
A DIRTY BATH Recently I watched a house sparrow dusting by the railway track in the city of Chicago. The dust may have been in part "clean" earth, but in part it was soot, city dust, and soft-coal debris. The sparrows here were dingy, all had their plumage heavily impregnated with city grime, and looked very different from the sparrows in the country. And this sparrow I was watching when it had finished dusting was the worst of the lot. These city sparrows, even when they bathe in water, seem never to get much of the grime out of their feathers.
This reminded me that Oscar Heinroth once wrote that birds do not bathe to get themselves clean, but bathe as an aid in bringing their feathers into order and making them lie smoothly. Perhaps he is right. Certainly my sparrow did nothing to clean himself.
It is in arid countries, plains and deserts especially, where many of the birds take only dust baths. In more humid regions water bathing is the rule. But some birds do both, like our flicker and our house sparrow, bathing now in water, now in sand.
In northern climates, when the land is held in the grip of winter, the water frozen over, and the earth covered with snow, neither dust nor water bathing is possible. Then, it has been recorded, some birds find a substitute in snow. Among other cases, in Alaska the hawk owl has been seen to perch in the snow on the tops of telephone poles, and go through the motions of bathing; in England a rook was recorded as bathing "in crisp powdery snow as if it were taking a bath in dust or water"; and in New England in midwinter juncos have been recorded bathing "in light dry snow, just as other sparrows take dust baths in hot weather."
The snow evidently is used as a substitute for dust in these northern latitudes.