Not exactly clear, here's what the national bluebird folks have to say:
Tony M
Typically, old nest material is removed when the breeding season is over. New evidence suggests that this may not be the best procedure. After nesting season is complete, some people elect to seal off their birdhouses to prevent unwanted winter tenants. Some people render the boxes unusable by propping the door open. Others leave the boxes as they are and allow them to be used as roosting places for birds, mice, or squirrels. If your nest is soiled with fecal matter, we recommend removing the nest and cleaning the nest box out with a solution of 1 part bleach to 10 parts water. If the nest is not soiled, you can choose to leave the nest in place or remove it altogether.
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