Metamorphoses Book 5: Minerva Meets the Muses at Helicon
Athene (Minerva) left her watch over Perseus and went to Helicon to see the Muses. She wanted the sisters who inspired song, poetry, drama, and other art forms to show her the spring that had issued forth from Pegasus' hoof. The Muses took her to he beautiful stream that was said to inspire poetry, and Minerva was impressed by the beauty of the Muses' home. As the goddess of war talked with the Muses, nine magpies gathered near the window. Another of the sisters told Minerva the story of the Pierid sisters who challenged the Muses to a talent duel. The deal was that whoever lost the duel would leave their sacred spring. Nymphs were chosen as judges, and one of the nine Pierid sisters went first.
The Pierid song was about the battle between the gods and the giants, and she sided with the giants, calling the gods cowards for disguising themselves. That's not such a good idea.
Calliope, the muse of poetry, went first for the Muses, and the song she sang was about the rape of Proserpine.