Mystery egg

I think it's a blackbird's egg, and it's on the ground under a hawthorn bush. I can't see a nest in the bush, but it is quite thick at the top, so can't be sure. I'm wondering if it was predated, then dropped. 

Update - 22 May.  I put a CCTV camera onto the egg to see what would happen.  The first visitor was a blue tit, but it didn't stay for long.  Then passing pigeons stepped on it and that opened it up and allowed the slugs to visit. Finally, this morning, a sparrow picked up the shell and  flew off with it.