Its called mobbing. Putting too many animals in a small space. They eat the grass too nothing and fertilize the ground. I seen where others were using chickens to work in their gardens. Not a weed one is growing in their pen, but when I turn mine in the garden they dig up plants and don't always eat the weeds. I saw a plan for a moveable collapsible chicken tractor herehttp://craftlog.org/craftlog/?p=4175
So I built one. I used plastic bird net so it is lighter I also didn't make a separate side panel because of the plastic net. The chickens can get out the sides if I don't put something in front of the sides. I move them up and down the rows and they weed, loosen soil, fertilize and work it in, and they find and eat all the grub worms. So far I am very pleased.
My only problems are putting the chickens in an taking them out. The garden is a short walk from their pen and I can't open the door and tell them to go and get in the chicken tractor and get to work. For some reason they don't listen to well.