Which way do the boxes face? They won't nest [usually] if they are going to get too hot.
DH made a sparrow terrace - taken over by 1 gt tit pair

4 the last 3 years.
The single box has blue tits in every year - these face North/East
One year a robin nested in a plastic sack that was filled with compost, how the cats never found that and another year, right in the back of the greenhouse - how the nestlings weren't roasted in their bed I don't know!
More spawn laid last night

even on top of the pond pump
