Our nest box has been used for a fifth successful year. This year we had a camera so can see all the lovely activity. There were 9 eggs. For the last few days a chick has died overnight. This morning the last 3 are dead. The hatchlings were 16/17 days old(they hatched 2 weeks last on Thursday). Both parents have been coming in and out with lots of food. What could have happened? Had they been fed something bad?