I found this on davesgarden.com
I was wondering about the different experiences people have been having with styrofoam and their chickens.
Mine attacked my homemade hatcher like ice cream when I left it out to sterilize in the sun. So I could see why it might be a good thing to use to distract chickens from plucking each other.
Other people report their chickens were seriously sickened or killed by eating it.
I think the confusion may be from the general term "Styrofoam". It is a registered name for the insulation "polystyrene". Polystyrene is the substance they make coffee cups and disposabel fast food containers from (or did). It is also what they make the styrofoam coolers you can make into incubators. It isn't, in itself, considered toxic (although some of the chemicals used in its manufacture are) but it takes forever to breakdown and is not very environmentally friendly for that reason.
Unfortunately, polystyrene is highly flamable and is usually not approved for use as insulation in building construction. Solid building insulation is made from "isocynates". Isocynates are very toxic and can have many long term health consequences. See this link:
http://www.dhs.ca.gov/ohb/HESIS/iso.htm
So I wouldn't panic if your chicken attacks a styrofoam coffee cup--but I would be very careful with them around solid building insulation.