Suggestions for cleaning birdhouses are from late autumn to late spring. Normally I do it at spring that there is something to keep them warmer during winter without having to put sawdust (or something else) there for winter. And spring is also good that you can put some new ones at same time, if you are putting new ones.
Typical nest box is standard box with about 32 mm hole in it. In those goes most common birds that use birdhouses, meaning different tits (great, blue, crested and willow) and pied flycatcher or sparrow. So this have become almost "standard" size in many cases. Those who take it more serious do custom ones for different kind of birds with variation all kind of sizes of box, including wall thickness etc.
So now couple of new ones are put in place.
Old ones are repaired and cleaned up.
And now one different size one is put up there in hope that some new bird uses it
Some "old favorites" are all ready taken and in use
