Blue tit chicks 13 down to one?

Hi its Taco here a new member. 

I have a camera in a blue tits box and was delighted when we watched blue tits building a nest. Then to our amazement 13 eggs were laid and hatched all approx over a couple of days. Every day we have watched the 13 chicks get less and less to where we are today with only one chick left. Why has this happened do you think? In the beginning both pair of adults came back to the nest with plenty of caterpillar bugs etc but this did slow up. I was expecting a few chicks not to survive but only one chick left. Mother still feeding this one chick.

Regards Taco. Paul Partridge