When do you clean out bird boxes if birds roost in them over winter? I have 3 wrens roosting in a bird box which is full of nest material from last year. The wrens built a lovely mossy nest last spring and then disappeared. Then a hornet took over for the rest of summer. The wrens came back in the winter, pulled out the hornet nest to make space and have been there overnight ever since. I’m not sure whether to remove the old nest material or not. It’s only a small birdbox and the nest completely fill it. The grass & moss comes all the way up to the roof so you can’t see anything if you lift the lid.
Not sure what design of box it is. Regardless, I personally wouldn't be cleaning that one out. With no evidence of it being used for breeding prev., there's every chance it won't be used for breeding this year either. Always best, IMO, for any box cleaning to be done in Autumn.
Thank you for your reply. It’s an ordinary box with a small round hole. I think I left it late last year because I wasn’t sure how late in summer/ early autumn they may still nest. What’s a late brood? September? Then suddenly it seemed to be winter and I thought I’d leave it until Christmas or after and now they’re roosting. I’m new to this and I’ve not been aware of roosting in boxes before. I just thought you could clean the boxes anytime over winter. I’ll do them around October next time if that’s best.
Small round hole suggests aimed at blue tits. As they only nest once a year, there won't be a late brood. In the unlikely event wrens do breed in it, you would need to leave it later in the year, but Oct sounds the ideal time.