Dead baby swallows

Hello, 

I had a really upsetting experience this evening. I walked into the horse’s stable just about to go and get them in for the night. When I noticed 4 dead baby swallows in the floor under the nest.

The nest hadn’t fallen and didn’t look damaged. I couldn’t see the parents. So very sad when the parents have been actively feeding the babies every night. 

Does anyone know what could have caused this? So so sad.