I'm still trying to figure out how they did this.

In the old coop the bottom floor is 1/2" wire mesh, even on the ground under the coop.

Rats burrowed under the coop trying to get in so they could eat the feed. The chickens killed two rats that found a way into the coop itself. But the rats had discovered that if the tunnel was in the right place they could collect feed that was dropped by the hens. Since the feed was then on the floor of the rat tunnel the chickens could no longer reach it through the mesh.

I found a dead and dried up rat in this tunnel. Chickens will kill rats, but how did they kill a rat through the wire mesh?

The meanest rat hunter in the flock is Frazzle, the brown hen on the lower left-center.