Title is basically self-explanatory, but crows are able to peck at crops even when separated by a fenced-off, roofed area, so long as there's less than 1 full block separating them and the crop. For instance, fences, which aren't a full block wide, allow crows to peck at crops through them. That also applies to wall blocks.

They can't pick up the crops once they've picked at them, but they can break any adjacent to the fence. Also, it seems like their spawn rate is really high compared to some of the other mobs, though maybe that's just me?