Tree Swallows (Tachycineta bicolor) commonly line their grass nests with feathers of other species. In one of three years studied, there was a significant negative correlation between numbers of feathers in the nest and chick nestling periods (i.e. broods surrounded by more feathers fledged earlier). In the third year of the study, the population was divided into two groups, balanced for all measurable aspects of parental quality. In the "removal" group, all feathers were removed daily and in the "control" group all feathers were allowed to remain. All nests and their contents were disturbed equally. Chicks in removal nests had lower growth rates (in mass, wing chord and tarsus) and higher infestations of mites and lice. I suggest that feathers aid chicks directly by preventing hypothermia and indirectly through higher growth rates by allowing earlier fledging when necessary. Protection from ectoparasites may be an important advantage in natural nests where nest cavities are not cleaned out annually. Inadvertent removal of feathers from nest boxes may be an importnat cause of posthatching declines in feather numbers.

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