Ἦροσ ἄγγελοσ ἰμερόφωνοσ ἄήδων. [transcription]
The messager of spring, the sweet voiced nighingale.
Quoted by the Scholiast on the Electra of Sophocles, 149, "the nightingale is the messager of Zeus, because it is the sign of spring."
Compare Ben Johnson's "The Sad Shepherd," Act II, Scene vi: "The dear good angel of the Spring, the nightingale."