I generally clean out the boxes that have nests once a year, usually in Sept., running a bit late this year. By cleaning out in the late summer, it allows the nest debri to dry out (gloves and dust mask highly recommended), and reduces the risk of interrupting a roosting owl which may scare them away. Nest debri when dry isn’t too bad, kinda a mix of wood chips, feathers and dried ick. With the boxes that are unused or used just as roosts, its a check and see, sometimes starlings remove the wood chips, so you are left with nothing much on the bottom, other times things look pretty good, maybe a few pelletes and there is no need to clean it out.
Well I was hoping the larger ‘slot’ opening would be less desirable to starlings, but alas starlings like it just fine. Took out a 3/4 nest that the starling built.