One reason for doing this is to remove any remaining nest parasites, thereby supposedly preventing parasitism in future nests. Fourteen percent of the nest boxes in which the previous nest had been removed contained blow flies. The figure includes blow-fly data reported for 24 species of birds. This figure combines information for 24 cavity-nesting species. These results are preliminary.
Many more questions remain to be answered about birds and blow flies.
It may be that cleaning out nest boxes decreases blow-fly parasitism for Eastern Bluebirds but not for Tree Swallows, or vice versa. Look for more Birdhouse Network results in future issues of Birdscope .