What is happening in that last poem? You can read a more comprehensive summary and analysis here, but the short(er) version is that Yeats is drawing from a Greek myth in which Zeus, in the form of a swan (he loved to take animal forms and do weird shit to women) either rapes or seduces (uhh) the human, mortal Leda. Helen (as in, of Troy) was born as the result of this union. Leda’s daughter with her mortal husband, Clytemnestra, married Agamemnon. Helen and Agememnon were catalysts of the Trojan War and Agamemnon ended up being killed by Clytemnestra’s lover. So, a lot went down because of Zeus’ aviary interlude.
Yeats is using the violence of this myth to say something about the colonial and oft-violent relationship between Ireland and England. But that is for a whole other post.
The myth has also inspired countless paintings, sketches, and sculptures over different periods and in styles.
Images from Wikipedia.