I realize there are better times to clean birdhouses, but like most homeowners and gardeners I do what I can when I remember to do it. The wisdom on bird house cleaning seems to be after each family raises its young and they all leave the nest. If your remember to inspect each house for vacancy and then clean, then you are a far more attentive birder than I.
Store and Supplies
My reminder to clean birdhouses here is to watch for sales at department stores for spring cleaning supplies. When I see all the brand names of bleach, window cleaners, disinfector sprays,and paper towels, I know it is time to go into action. Hopefully this will happen before the local bird population begins to have sex on the brain.
Why clean the houses? Simple answer is more birds are attracted to clean, empty houses than ones filled with last years nests. We all want to see more birds using the houses we put up for their use and our enjoyment. Also, but cleaning the houses we prevent pests and diseases. A good cleaning usually removes any mice that may have moved in over winter along with insects. Bacteria can breed in the house long with fungus, and feather mites are deadly to the young hatchlings.
First up I recommend a pair of gloves and a trash bag for the debris. Take down the house, open and remove all loose nest material into the trash bag for disposal. Any crusted material can be scraped into the bag. I use a spray bottle of bleach and water mix (1 part bleach, 10 parts water) to thoroughly spray inside every nook and cranny of the house. Set the house in the sun to dry and later return to put the house back up for occupancy.
Dont risk your birds walking your driveway with picket signs saying Slum Lord! Below Standard Housing. Unfair to Birds. And that is a lot for them to carry on a sign. Remove any houses that have seen their day and usefulness. If pests such as raccoons have accessed the nest box, then perhaps a higher mounting would be best for protection of the eggs and young. If the wrong birds are using the box, perhaps a move before nesting begins would be in order. To be an equal housing landlord you need to have the right housing in the right locations.