It is important to clean nest boxes out after the breeding season has ended. Some birds use the nest boxes as roosts in the cold winter days and nights.
Some birds stay for 16 hours a day in their roost – which could be the nest box that you bought
Here is a link to an article which gives some of the reasons why –
I was reminded how important it is to clean nest boxes out when I saw two sparrows trying to get into a nest box that had been used as a wasps nest this summer!
I wish I could have got a photograph of the sparrows. They kept poking their beaks in and then flying away.
We left the wasps nest alone during the summer as it was not doing any harm. It was out of the way and the wasps never came near to the house.
Please read the article above about cleaning out nest boxes. It isn’t the nicest job, but it helps birds survive the winter