Okay, mystery solved. I have bagworms, or tent caterpillars in the peach tree, so now I know why the chickens were after the
trees.
I am not aware of bagworms spending any time in the ground though at all, but it seems the chickens really think these worms are in the ground too.
I covered the soil, and when I removed the boards, I found many dead bagworms. Aha.
Is there a plant guild that would stop these beasties from getting on my trees?