It's important to clean a birdhouse at least once per year. While some birds will return to the previous year's nest, or will construct a new nest on top of an existing one, it's better if they make a clean start. Old nesting materials can harbor parasites, insects, or disease which can harm the nestlings. Also, when nests are built on top of one another it means the baby birds are higher up in the birdhouse than they should be. This is unsafe because they may tumble out of the opening before able to fly, and also because they are more accessible to predators, particularly other birds, who can easily attack them from the perch.
Before taking the birdhouse down, make sure there are no birds inside. Position yourself to the side of the house and tap it, observing for movement within. Look inside if you can, but stay away from the front in case a startled bird, or other critter, exits quickly. Once you're certain that it's unoccupied, you can safely take it down.
Most birdhouses are designed to open easily, with a hinged roof, a sliding side, or some other mechanism. You may need a screwdriver to gain access to the interior, especially if it's a homemade house. Wearing gloves, open the birdhouse, and tip out the contents into the plastic bag. Tie the bag closed and discard.
Brush away any debris clinging to house, inside and out. Sweep out all nesting particles, and scrape away dried droppings. Next wash the interior with the bleach solution. Include the entrance to the house, and the perch, if there is one. Rinse with clear water and let dry in the sun.
Once the birdhouse is dry, inspect all hardware, fastenings, and hinges. Make sure everything is in working order, and tighten any loose parts. Check that the roof is secure and sufficient to keep out the weather. Make sure any drainage or ventilation holes are clear. Finally, look over the entrance, perch, and all interior surfaces. Remove any sharp edges, such as protruding nails or splintered wood, which could harm the nestlings.
Close the birdhouse and place it securely back onto its stand. The birdhouse should be well off the ground, high enough to be safe from cats and other predators. Face it east or southeast, for good morning exposure. It's helpful to locate the house within a few feet of a bush or small tree, so that fledglings will have a place to land when learning to fly.
The birdhouse can be cleaned before nesting season has begun, and again in the fall after all young birds have left the nest. Don't clean the house mid season, as you may inadvertently disturb an active nest. If you're unsure whether a birdhouse is in use, it's best to wait. You can leave the birdhouse in place all year. It will provide shelter for birds that stay for the winter.