We had birds (blue tits) at who work last year nested in a metal wall mounted cigarette bin, this year we provided a hand-made wooden bird box specifically designed for blue tits and then blocked up the larger holes on the cigarette bin.
The young have since fledged, which is great and you can no longer hear anything inside the box. What time of year should we clean the box out, if at all? Reason to ask is one young bird last year did die (the rest fledged) and was left by the parents however the cigarette bin gets cleaned out by maintenance staff, so the point was moot at the time. This box is not maintained by the same staff but by my work.