This occurred two days ago, as we were piling into the car for the school run...
Maddy: Mum, mum, come here quick
Me (slightly hassled): What? What are you yelling about?
Maddy: There is a big crow with one of our eggs in its beak!
Me: What! Are you sure?
Anyway... after a bit of a google search... yes, crows and ravens do raid nesting boxes and steal chook eggs.
Mmmmm... we have to devise a plan of attack now.