The Bonny Birdy is Child ballad 82 (Roud 3972).
Synopsis
A knight is riding when a bird asks him why he is about so late and tells him his wife is with her lover. It had been a wild bird until the lover caught it and gave it to his love. She did not feed it well, so it is telling her story. It flew with the knight to her bower, and sang of how the lover should be away. The lady asks what reason there is for him to leave, and the bird sings that a man in bed with another man's wife should always leave quickly. The knight enters the bower and kills the lover.
References
The Bonny Birdy Wikipedia(Text) CC BY-SA