A lot of people don’t realize you need to clean your birdhouses on a regular basis. A good time for doing this is fall when you know migratory birds have left and breeding season is over. Of course this varies geographically, so you should probably do some Googling of the species of birds that you have in your yard to be sure that you won’t be disturbing actively-used houses.
There are some rare exceptions where you should NOT clean birdhouses… but in the vast majority of cases it is a good idea, especially if predator birds are present, because if birds have to build on top of existing material, the brood may be too close to the entrance within easy reach of predators. For a full discussion of this, I would check out this article at NestWatch.
The cleaning process is pretty simple, just make sure you don’t use any chemical cleaners, as this may be toxic to birds. Check out the following video for some tips and tricks on birdhouse cleaning!