Since posting about Eastern Bluebird nesting preferences on my blog, I have
learned that the presence of bluebirds for boxes containing old bird nests
which might contain parasitic wasps is not because the birds eat the wasp
larvae, but rather, because the wasps parasitize bluebird blowfly larvae,
which suck the blood of nestlings and weaken them. The infestations of
blowflies can run into the hundreds.