Try a bird-scarer, like old CDs hanging on canes, or a child's windmill or two. It might keep them away - are they after leather jackets in your lawn?

That's exactly what they're after, leatherjackets, so they'll stop soon, as the last of the big ones is eaten, or emerges as a crane fly. Phil

And just a quick point, they are more likely to be rooks than crows. Remember a crow in a crowd is a rook and a rook on its own is a crow! (Unless they are Jackdaws of course ).