Every year in Feb, I clean my bird houses, omitting the ones in use by flying squirrels. I will reclean the ones Blue Birds use as they do more than one nesting each Summer. A little soap and bleach does the trick.

Yesterday I did repair and staining, today I mounted the ones I had to take down. All in all, have about a dozen I can see from my chair, more other places.
I spray them with a mixture like found in Frontline Pet flea killer.

The good news is I did not fall off the ladder this year.

Now it is your turn to get out there and clean yours. Birds start to search and choose as soon as warming begins.