I've never really thought much about the term 'swan song', until today. I was going to use it in an article, but googled it to make sure I was using it in the right context.

According to Wikipedia:

"The phrase "swan song" is a reference to an ancient belief that the Mute Swan (Cygnus olor) is completely mute during its lifetime, until the moment just before it dies, when it sings one beautiful song."

A little more digging reveals that these types of swans are not entirely mute, but they are less vocal than other swan species.

Still, what a cool legend.