Swans are fascinating. It's their unparalleled beauty and their graceful form that has made them a part of many folk-lores from different cultures across the world. They are an epitome of purity and for those who did not know, swans mate for life. Sigh!!!!
The expression, Swan Song has its roots in a myth.......a very poignant myth. It was once believed that upon death, the otherwise silent Mute Swan (in reality, it's not a completely silent bird, only less vocal than other swans)would sing one achingly beautiful song just before dying. This legend was well-known to be false as early as the days of ancient Greece, when Pliny the Elder refuted it in Natural History, AD 77. The legend stayed though to give us the expression.
Trivia: Socrates' last words before being put to death in 399 AD: "You think I cannot see as far ahead as a swan. You know that when swans feel the approach of death they sing, and they sing sweeter and louder on the last days of their lives because they are going back to that God whom they serve." (Plato)
Sources: The Usual suspects :)
http://www.answers.com, http://en.wikipedia.org/, http://www.phrases.org.uk/
Pic Courtesy: Google Images