a final performance or effort (especially before retirement)

a reference to an ancient and false belief that Mute Swans (Cygnus olor) are completely mute during life, but sing one heartbreakingly beautiful song just as they die

"Swan song" is a reference to an ancient belief that the Mute Swan (Cygnus olor) is completely mute during its lifespan, but may sing one heartbreakingly beautiful song just before it dies. However, it has also been known since antiquity that this belief is false; "mute" swans are not actually mute during life – they produce snorts, shrill noises, grunts, and hisses – and they do not sing as they die. In particular, Pliny the Elder refuted the belief in A.D. 77 in his Natural History (book 10, chapter xxxii: olorum morte narratur flebilis cantus, falso, ut arbitror, aliquot experimentis, "observation shows that the story that the dying swan sings is false").