Nothing - you could try slug pellets to keep slugs off, or it could be earwigs, but no spray deters those.

Nothing that I am aware of, probably bird pecking them.

You can deter birds by stretching lengths of black thread across at intervals above the plants..

I'm sure it's earwigs. I have the same problem even in the baskets and it seems that it's the white ones that they go for so I would guess there is little that you can do.