In a cage I used to have quail in I planted some ground juniper its evergreen it doesnt get very tall which is nice in a short cage the quail dont mess with it so they wont kill the plant or anything like that. One thing I liked about the juniper is that I positioned it as a wind blocker to give them a bit of extra protection from that northern wind in winter. Other than that I would think tall grasses would be good but Ive never had a chance to try that myself. Goodluck!