oyster shell?

I don't think chicks are supposed to eat oyster shell, as this can cause bone deformities. They are supposed to have grit

 

They're just trying things, like little kids putting things in their mouths, they'll stop when they figure out they don't need it. In the meantime, just as long as they're getting outside where they can get to dirt, they'll be getting grit. If no, you may have to give that too if they're being feed something other than starter.