When Wart is changed into an owl and a goose by Archimedes and Merlyn, what lessson(s) does Wart learn, in T.H. White's The Once and Future King?
In T.H. White's The Once and Future King, Merlin sends his owl, Archimedes to turn Wart into an owl. In this form, Wart learns the physics of flying and learns, also, to see the world from a completely different perspective, as owls see differently than humans do.
One important lesson that Wart learns as an owl is a point of honor as far as these birds are concerned. To fully understand, please recall that in Chapter Three, Merlyn was already very clear about what a noble and wise bird the owl is. He told Wart:
...you will learn that owls are the most courteous, single-hearted and faithful creatures living. You must never be familiar, rude or vulgar with them, or make them look ridiculous.
In Chapter Eighteen, then, Archimedes adds to what Merlyn has said, explaining another way in which owls are principled creatures. After Archimedes has caught and eaten a sparrow, Wart—"inclined to be blood-thirsty"—asks if he might also do the same thing. Archimedes tells Wart that he may not. He says...
...no owl kills for pleasure.
This bit of information would indicate that killing for sport is not what a noble creature would do—this is one lesson Merlyn has for Wart. It is echoed again later when Wart is turned into a goose (meeting the "Wild Geese") and encounters Lyo-lyok, one of the Wild White-fronted Geese. After Wart puts in his time guarding the flock while the others feed, he asks Lyo-lyok about why they need to be vigilant: were they worried about an attack from other geese?
At first Lyo-lyok does not comprehend what Wart is saying, but as she begins to realize that he refers to the killing of geese by other geese, she angrily responds:
...of course there are sentries. There are the jer-falcons and the peregrines, aren't there: the foxes and the ermines and the humans with their nets? These are natural enemies. But what creature would be so low as to go about in bands, to murder others of its own blood...
There is not fighting, she explains for...
There are no boundaries among geese...Those ants of yours—and the humans too—would have to stop fighting in the end, if they took to the air.
(This refers directly to the theme/concept of "war" in the novel.) In order for Wart to learn to be noble as a human, he must understand that there should be no killing for sport, and that there is something unnatural about creatures that murder others like themselves. These are the lessons Meryln wants Wart to learn.