Botulism is caused by eating a toxin produced dead animal or fish remains, or vegetation buried underground or underwater without oxygen (anaerobic conditions.) Moldy feed can cause mycotoxin poisoning, and have some similar symptoms. Here is some reading:
https://www.thepoultrysite.com/disease-guide/mycotoxicosis
https://www.hobbyfarms.com/botulism-poisoning-in-chickens/
https://www.thepoultrysite.com/disease-guide/mycotoxicosis
https://www.hobbyfarms.com/botulism-poisoning-in-chickens/