Cleaning out. Birdboxes

Hi everyone 

 i usually clean all my hird boxes out this time of year . The house were i live now seems to attract alot more of a variety of birds , one of my birdboxes this year was occupied by tree sparrows , for some reason they built a complete nest then never bothered layyng eggs in it , i can only assume it was because starlings went though a Stage of investigating it which mist  of put the sparriws  off .

 The past few days birds have been attending this box , my question is do i leave the box as it is so birds have a nice  warm roost or shouod i still just empty it and clean it out 

 the nest looks so comfy haha