I had 3 nestboxes in the garden all used by Blue Tits this year. The constant to-ing and fro-ing of the increasingly ragged looking parents, followed by the louder and louder cheeping of the chicks, which developed into chirruping and calling has been delightful to experience.
The 3rd box was started later than the other two. I came upon it by accident, whilst gardening. I thought the box was unused as the lid had slipped off, so I went up to it to straighten the lid, looked inside and saw a gaggle of tiny orange mouths waggling around! I was stunned and so delighted, but really worried that I'd frightened the parents off and that they would abandon the nest.
I left it for a couple of days then went back to check, and saw one dead chick and a few more live ones. I was gutted, I really thought my presence had lead to their abandonment. So I was delighted when I saw the parent bird going in and out.
I gave the box a very wide birth - its a bit low down and so it's not in the best place, we put it there to replace an old wren nest which we found when removing the ivy, so we hoped the wren would come back to it. I had to neglect my weeding from that area because whenever I was near I'd see the parent bird chirruping at me to move away!
Watching from afar, I realised that there seemed to be only one parent feeding the nest. Both my other boxes had a very efficient tag team of parents, one would leave and another would enter the box immediately after. But on this little box, I only ever saw one doing all the work 
Still, they carried on, and I could hear the chicks cheeping from the box. My two other boxes fledged happily, so I couldn't resist one more look in the box and saw a bundle of healthy chicks about a week off fledging I thought.
(I must say here, I was always very careful not to disturb the parent. Of the 3 times I checked the box, the first was accidental, and the other two I would wait until the parent left before quickly looking in then backing away so as not to deter her from coming back).
Anyway, all was well with the parent busily collecting catterpillars from the garden and busily attending her chicks. I went away for a few days last week, and when I came home the box was silent.
I watched it for a while, but no cheeping, and no parent. I looked inside to see a box full of almost fledged dead chicks
I think the parent must have been killed. She was doing all the work alone. Those chicks must have starved.
So I'll have to remove them from the box. I've been putting it off, but it has to be done or it will become stinky and no one will want to nest in it next year. I'll make a nice little resting place for them, and I'll probably cry
Rest in peace my little Blue Tit chicks and their hard working parent.