"Sing my precious little golden bird, sing! I have hung my golden slipper around your neck." - The Nightingale
This episode is based on "The Nightingale," a fairy tale by Hans Christian Andersen. In the tale, the Emperor of China learns that one of the most beautiful things in his empire is the song of the nightingale. When he orders a nightingale brought to him, the Emperor is so delighted with the bird's song that he keeps the nightingale in captivity. When the Emperor is given a bejeweled mechanical bird, he loses interest in the real nightingale, who returns to the forest. The mechanical bird eventually breaks down due to overuse, and the Emperor is taken deathly ill a few years later. The real nightingale learns of the Emperor's condition and returns to the palace, and Death is so moved by the nightingale's song that he departs and the emperor recovers.
Like the nightingale, the Seltenvogel is a beautiful and rare creature that's kept captive because of its value. But instead of song, the Seltenvogel provides a different asset: they are known most famously for producing an Unbezahlbar once in their lifetime, which resembles a large golden egg.