Swans are beautiful and pleasing to the sense of sight but they are not musical like cuckoos. They make sounds but hardly do they pour out their melody that provides food for your emotions. This has led to a popular legend: a swan is capable of singing a song that is as beautiful to the sense of hearing as it is to the sense of sight. People, very few in number, hear its music, because it only sings only once in its lifetime, so the idea is that if you come across a swan that is about to meet its end, it lets out the most surprising sound that you have ever heard.
Taking this fact into account, the last work of an artist, maybe a book or anything that offers you perennial value can be considered their swan song that speaks volumes about their golden career or any lifetime achievement they have had to their credit in their chosen field of activity.