If they are free ranging chickens it may have nothing to do with what you are feeding them. They may be getting something when they are “out and about”. Check the list of plants that are toxic to chickens to see if you have any of them growing in the area they have access to. Also make sure the water they drink is clean to make sure there is no bacteria. BTW if you just got them it may also simply be a change in diet.
My New Hampshire Reds got into a Poke Berry bush here in Georgia (safe for birds) and the hen house looked like a blood bath; red diarrhea everywhere.