I hope you're able to find out more about the condition your birds have been suffering with. I mentioned TSC because their feed has obviously been tampered with and is causing a country-wide situation of chickens not laying.
Do you possibly have a sense about whether your chickens are intentionally shaking their head as if to get rid of something, or whether their head is involuntarily shaking as if a nerve symptom (neurological)??
You could contact www.BeyondPesticides.org 202 543 5450 for information. I sadly left Florida because of the aerial and ground pesticiding, which my parents and I each had serious reactions to. I used to routinely pull my bucket herb garden inside during pesticiding and, had I had chicks, would...