Tree Swallows - an Empty Nest

Well, the tree swallows were in their nest yesterday. Today is day 21 in their little lives, and this morning they're gone. At least, five of them flew. A sixth didn't see the outside world. It laid motionless in the nest this morning at 8am, dead. I cleaned the nest out of the house and left the front open for the sun to him for a few days...