Bird Lovers, Help – Tragedy!

We have three full bird houses in our backyard; one with baby blue birds, another with baby sparrows, and the last with baby tree swallows. We’ve been checking them every day and watching them grow. However, this afternoon we went out to check on the blue birds and came upon a terrible sight; All five baby birds were dead. They all had their feathers and were just about big enough to leave the nest. They look perfectly intact, meaning, no signs of violence from another critter. Do you have any idea what could have caused this? We’ve witnessed both parents caring for and feeding the birds; Of course today, there’s no sign of mom or dad. We have never witnessed this before and my 8 year old ds wants an answer to what and why this could have happened. All I’ve been able to do so far, is dry his tears. 😥