no obj.
1- (of a bird) sing softly and with a succession of constantly changing notes（鸟）啼啭：
larks were warbling in the trees.
云雀在树上啼啭。
1.1
- (of a person) sing in a trilling or quavering voice（人）用柔和的颤声唱：
he warbled in an implausible soprano.
他用柔和的颤声唱出难以置信的女高音。
- a warbling sound or utterance鸟啭, 颤声。
词源
late Middle English （as a noun in the sense 'melody'）： from Old Northern French werble (noun)， werbler (verb)， of Germanic origin; related to WHIRL.