Mystery egg

We were trying to clear the overgrown grass between the shed and the greenhouse and then Statix came across this egg! It's been a mystery all day, we cannot see a nest in the eucalyptus tree above and it appears to have a hard shell so we think it's a wood pigeon egg but stopped what we were doing in case it was an Adder egg!

Its also been advised that it looks like a chicken egg! I think it must be recently placed there as I've stepped all over this area when we were taking down some eucalyptus branches.  Another friend also advised that foxes can take eggs and hide them.

All very odd! It's about 4-5cm and quite spherical. 

And I've been feeling a bit ropey all day after failing to drink enough water last nighy!