Mute Swans are not actually mute during life – they produce snorts, shrill noises, grunts, and hisses – and they do not sing as they die. Peterson et al. note that Cygnus olor is "not mute but lacks bugling call, merely honking, grunting, and hissing on occasion." This folktale has been contested ever since antiquity. In 77 C.E. Pliny the Elder refuted it in Natural History, stating: "observation shows that the story that the dying swan sings is false."
Nevertheless, the folktale has remained so appealing that over the centuries it has continued to appear in various artistic works. Aesop's fable of "The Swan Mistaken for a Goose" alludes to it: "The swan, who had been caught by mistake instead of the goose, began to sing as a prelude to its own demise. His voice was recognized and the song saved his life." Ovid mentions it in "The Story of Picus and Canens": "There, she poured out her words of grief, tearfully, in faint tones, in harmony with sadness, just as the swan sings once, in dying, its own funeral song."
By extension, "swan song" has become an idiom referring to a final theatrical or dramatic appearance, or any final work or accomplishment. It generally carries the connotation that the performer is aware that this is the last performance of his or her lifetime, and is expending everything in one magnificent final effort.