Nightingales are a type of song bird, known for their beautiful songs.
Background
While the nightingale is a somewhat plain bird in appearance, its song is thought by many to be beyond compare.
When King Graham entered Hagatha's cave, he found a nightingale in an ornate cave. Graham rescued the creature from the witch after first covering the cage with a cloth to muffle any sound it might make as they were leaving. When the King eventually arrived at Milvia's Olde Antique Shoppe, he discovered that the bird had been stolen from the woman by the witch. For returning the bird to her, Milvia gave Graham the brass lamp he needed to obtain the second key to the magic doors.[1]
Sing-Sing is the nightingale belonging to Cassima.
There once was an emperor who so admired the song of the nightingale that he had one captured and caged for his enjoyment. So much did he like the song, and so much did he dislike the bird's plain appearance, that the ruler had a jeweled, mechanical nightingale constructed--one with a song as lovely as the real thing. In time, the emperor grew ill and was told that only the song of a nightingale might save him. Alas! The song of the jeweled bird didn't, so he sent for the plain bird he had rejected. The nightingale agreed to sing for the emperor in return for a promise never to be caged again. The emperor agreed, and the bird's marvelous song, the most beautiful in the world, saved his life, and the bird was set forever free.[2]