I have had blue tits in box in my garden for approx three weeks and a lot of activity with food going in and out of the nesting box. However, I woke up the other morning to find no activity with regard to going in the box, so thought they had fledged. But no sign of any babies, but the parents come back periodically for food, which I had previously hid in the bush close to the nesting box to help them out. Do you think the babies died?