a little bird told me

Etymology:

• The phrase has a somewhat contentious origin: some attribute it to Ecclesiastes 10:20 "Do not revile the king even in your thoughts, or curse the rich in your bedroom, because a bird in the sky may carry your words, and a bird on the wing may report what you say." Another explanation is a simple allusion to carrier pigeons or other such messenger birds.
• John Heywood's Proverbs gives an earlier expression "I hear by one bird that in mine ear was late chanting", and characterizes "a little bird told me" as the 'modern' version.
• In a Norse legend, Sigurd slew the dragon Fafnir and got a bit of dragon's blood on his tongue when he was roasting its heart. This immediately made it possible for Sigurd to understand what the birds were saying, and what they were saying was a warning that Regin would not keep his word, but instead planned to kill Sigurd. This was borrowed by Richard Wagner's Siegfried, in which the main character comes to understand that the song of a small bird instructs him to steal a ring and helmet.
• It also may have come from an 1843 Hans Christian Andersen fairy tale entitled "The Nightingale". In the story, a small nightingale is put into the palace of an emperor for his amazing voice. When the emperor receives a bejeweled mechanical nightingale, everyone prefers it, because it is known what the mechanical bird will sing, while the real nightingale sings whatever comes into its head. As a result, the real bird is banished from the palace. One night, while the mechanical bird is singing the emperor to sleep, something in it breaks. He takes it to a watchmaker, who says that the cogs are worn down from too frequent use, so the emperor only plays it once a year. Unfortunately, the emperor soon falls deathly ill. He sees Death sitting on his bed, holding the emperor's crown, sword, and banner. Just then, the little nightingale flutters through the open window, and begins to sing. The song pleases Death so much that he agrees to leave the emperor to hear another. The nightingale sings again, and the emperor is cured. The emperor asks the nightingale how he can repay him. The nightingale says that he will gladly sing to the emperor of everything that is happening, far beyond the reach of his palace, but warns him, "You must not let anyone know that you have a little bird who tells you everything; then all will go even better."