Legend has it that the mute swan — a white swan found mainly in Europe and Asia, though it has been introduced into the Americas — is silent all its life. Then, just as it is about to die, it sings the most beautiful song.
It's just a legend, though. Mute swans actually make a soft snorting sound — it's not unpleasant, but neither is it particularly pretty. But the notion of its deathbed song has taken hold as a metaphor for a last act before the end.