A mediæval version of Ovid's tale about Coronis (Met. ii. 543, etc.). Phêbus; had a crow which he taught to speak; it was downy white, and as big as a swan. He had also a wife whom he dearly loved, but she was faithless to him. One day when Phêbus came home his bird 'gan sing “Cuckoo! cuckoo! cuckoo!” Phêbus asked what he meant, and the crow told him of his wife's infidelity. Phêbus was very angry, and, seizing his bow, shot his wife through the heart; but no sooner did she fall than he repented of his rashness and cursed the bird. “Nevermore shalt thou speak,” said he; “henceforth thy offspring shall be black.” Moral -
Lordlings, by this ensample, take heed what you say; be no tale-bearers, but `Wher-so thou comest amongst high or low, Keep wellthy tong, and think upon the crow.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894