What does "warble" mean?

Additional senses

1. 2.To sing in a trilling, quavering, or vibratory manner; to modulate with turns or variations; to trill; as, certain birds are remarkable for warbling their songs.
2. 3.To utter musically; to modulate; to carol. If she be right invoked in warbled song. Milton. Warbling sweet the nuptial lay. Trumbull.
3. 4.To cause to quaver or vibrate. "And touch the warbled string." Milton.
4. 5.To be quavered or modulated; to be uttered melodiously. Such strains ne'er warble in the linnet's throat. Gay.
5. 6.To sing in a trilling manner, or with many turns and variations. "Birds on the branches warbling." Milton.
6. 7.To sing with sudden changes from chest to head tones; to yodel.
7. 8.A quavering modulation of the voice; a musical trill; a song. And he, the wondrous child, Whose silver warble wild Outvalued every pulsing sound. Emerson.