Hi PP
You could try some pot eggs in the nest box, golf balls are a good sub though
![happy :)](https://chat.allotment-garden.org/Smileys/green/smile.gif)
The birds will hopefully see them and get the idea
![biggrin :D](https://chat.allotment-garden.org/Smileys/green/biggrin.gif)
Was the shell quite thin? That could be why, they will move them around using their beak so hopefully it was just a accident
![happy :)](https://chat.allotment-garden.org/Smileys/green/smile.gif)
Only other thing is could a crow or magpie been at it ?
SArah
![happy :)](https://chat.allotment-garden.org/Smileys/green/smile.gif)