I read about Worm Grunting tonight in National Wildlife Federation magazine, and did a Google, and found this information from Discovery. I found it very interesting.
Oct. 20, 2008 -- It sounds a little like snake charming. You drive a wooden stake into the ground and draw a flat metal rod across the top, creating an bullfrog-like grunting sound. Within minutes, hundreds of earthworms come to the surface, where they meet their doom in the buckets of fishing bait collectors.
Now, researcher Ken Catania of Vanderbilt University in Nashville, Tenn. has an explanation for the worms' hightailing it to the surface: Worm grunting mimics the vibrations made by burrowing moles, a voracious worm predator.
Catania, who studies moles, had read Charles Darwin's statement in a book he wrote on worms: "It has often been said that if the ground is beaten or otherwise made to tremble, worms believe that they are pursued by a mole and leave their burrows."
Others have observed certain turtles and birds tapping the ground to bring worms to the surface to eat.
"Then I heard about this 'worm grunting'," Catania said.