The bird consumed only plant and dairy/egg products and did not eat any kinds of meat.
Explanation:
This suggests a healthier animal, and, in theory, high quality meat overall. By having the bird take part in 'clean' vegetarian eating (which is different from vegan, where the bird would have eaten no animal products whatsoever), it suggests the meat would be better in taste and healthiness for the human consumer.
I think in this quote, Zeus is referring to the fire as the "evil thing in which they may take pleasure to their gut," because he refers to it right before he uses the word evil to describe it.
