warble
/ˈwɔ:bl; `wɔrbl/ v
(a) [I] (esp of a bird) sing in a continuous gentle trilling way （尤指鸟）叫出柔和的颤音
larks warbling in the sky 在天空婉转叫著的百灵鸟.
(b) [Tn] sing (a note, song, etc) in this way 用柔和的颤音唱（某音、 歌等）. warble, n (usu sing 通常作单数) warbling sound 柔和的颤音
the blackbird's warble 黑鹂婉转的叫声.