Hi Ellie, nest boxes should be cleand out later on in the year after nesting has finished. October to January time, if I am wrong I will be corrected by someone I am sure.
Beauty is in the eye of the beholder.
Hi Ellie, I agree with Catlady and you should leave the nest boxes until the end of summer (I usually take our nest boxes down at the beginning of October) before you take them down to clean them Blue Tits have one brood - here's some interesting info on them to give you some approx. timings for nest building, raising of chicks, etc, HERE
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Regards, Hazel