Many birds only nest once each summer but most of our residents attempt multiple nestlings. It is always a good idea to clean out bluebird and wren houses after each nesting if you houses are designed to do this easily. Once young birds leave their nest, they do not return. Cleaning out the nest is a good way to prevent passing one set of problems along to new residents. Bluebirds will not use the same nesting materials and will simply build a new nest on top of the old. If the old nest had an issue with lice or mites, those problems will make it hard on the new hatchlings. The exact date for fall cleaning is tough. Since many species will attempt a nest very late in the season if conditions are favorable, I would put off your “winterizing” until October or so if you can.