Here are a couple of links to review..r (@EggSighted4Life) mentioned checking the crop function. I don't know how to check this. What are your thoughts on this? My flock is not yet free ranging
https://the-chicken-chick.com/chicken-anatomy-crop-impacted-crop-sour/
https://www.backyardchickens.com/ar...w-to-know-which-one-youre-dealing-with.73607/
*Sometimes* crop dysfunction can be a symptom of Marek's. They don't have to be free ranging to catch it or at least I'm under the impression that it can spread for up to 3 miles or "carried on the breeze".
Any attack to the immune system may make other things more susceptible like Coccidiosis.. but you can test for that in a fecal float.
Corid works by mimicking thiamine (B1) to starve out and slow the growth of coccidia.. An overdose would most likely present as a thiamine deficiency..
https://www.merckvetmanual.com/poul...l stages of,-gazing, and frequent convulsions.