The story of Leda and Zeus (or Jupiter) was very popular in ancient Greece.
Zeus managed to seduce the lady Leda while he was taking the form of a swan which came to her in search of aid. As Wikipedia says: “Their consummation, on the same night as Leda lay with her husband Tyndareus, resulted in two eggs from which hatched Helen, Clytemnestra, and Castor and Pollux,” all of whom grew up to become legendary figures in their own right.
Images of their consummation were very popular decorations on ancient Greek stoneware….
This image is one of many story images of birds and animals featured in my calendar Legendary Tales: Myths and Legends
Ink drawing with digital color, 2005