I just built a run and it has tons of worms in it. I actually get down in the dirt and dig around in it with my chicks. They go nuts picking out the worms and showing off the ones they catch. Anyways I think my soil has so many worms because of all the recent rains and also it is in a shady location.
Just a thought but if you want to get worms you could try tilling in worm castings.