Back in 2003, I made a blue tit nest box and installed it in our Scots pine tree, about 15 feet off the ground (to discourage cats), away from the bird table (to give them a bit of privacy), facing north-east (to prevent the sun shining through the hole, or the prevailing winds blowing into it). Every February, I diligently clean out the box, disinfect it with boiling water, and re-hang it back in the tree.
So far, we have a 100% success rate. Blue tits have nested in the box every year since 2003.
I spent a quarter of an hour this evening watching an adult bird taking food into the box, then flying off again with the chicks' latest faecal sack in its beak. But after ten minutes or so, I started to get worried. As far as I could tell, I had only seen one parent bird, never two at the same time, which is unusual in a 10-minute stint.
I needn't have worried. It turned out that the missing parent was inside the nest box all the time, presumably keeping the chicks warm.