Page 751 has the dose for birds.It's a tapeworm. They get them from ingesting fleas. You may also find things that look like grains of rice. Those are tapeworm segments that have dried on contact with the air.
praziquantel is what you need to get rid of them. Sorry I have no idea of dosage for chickens.

http://www.armchairpatriot.com/Home...] 6th ed. - D. Plumb (Blackwell, 2008) WW.pdf