I have them under my feeder most of the winter, then the shrews come to hunt the voles. You can feel the spongy area under the grass after a few weeks of feeding the birds if you get voles moving in to feed.
A lot of grain is spilled to the ground by the feeding birds. They rake the seed back and forth with their beaks causing much loss of feeder seed being spilled to the ground.
Some areas may not get this behavior but I do here in SW Pa.
As Jim stated voles are primarily grass, soft tubular and smooth bark feeders but grain is good if they can find a source for it. Grain is much more sustaining nutritiously in cold temp conditions..
Well, that is what is going on under my feeder. The "picky" birds flip a ton of seed out until they get what they want. Underneath is mounded up like something is tunneling...but no immediate hole. And the grass around the bird seed is clipped pretty good.
The has been a vole hole about 15' away all summer. I've been messing with him, trying the small mole traps by the "gophernator" guy. Now I'm dialed in to the vole deal(I didn't realize it was 1 or more voles) and going to give 'em the rat traps covered with a down spout...lol