Expensive Birds proofing not effective!

Apodemus

Davesnave said:

... like the de-stoning machines the farmers use, if they worked, surely they wouldn't need to keep using them?

It might depend on how they are using the destoning machine. Around here, these are used for root crops. The stones aren't removed from the field but the output of the destoner essentially lays a french drain adjacent to each bed where the potatoes or carrots are planted. This has the combined benefit of creating a fine seed-bed, improving the field drainage and allowing a mechanical harvester to operate without every second potato being a stone.