Why haven't the birds eaten all the berries?

Calling all you Bill Oddie's out there.I've just noticed whilst walking my dog in the countryside that there are still lots of Blackberries to be had, some still red, not yet turned black.Why haven't the birds eaten them? I thought they loved berries.It's a pity to see them waste and rot.They are no good to me red, so I leave them in hopes the birds will find them.Will they?