They had developed a conditioned taste aversion to sheep.
Conditioned taste aversion is a type of classical conditioning in which an animal learns to associate the taste of a particular food with subsequent illness due to the consumption of poisonous or toxic substances in the food. An animal can develop a taste aversion when ingesting a particular food is followed by vomiting, nausea or illness. This trains the body of the animal to avoid further ingestion of the particular poisonous food in the future.