Answer: warble

AnswerCrossword Clue
WARBLECarol.
warblewarbled song
warbleto trill (to play with a tremulous sound)
warbleto sing with trills, as birds
WARBLESing
WARBLEIt's trilling
WARBLEImitate a prothonotary bird
WARBLEBirdsong
WARBLETrill.
WARBLEBird's song
WARBLESing.
WARBLEBirds do it
WARBLETrill
WARBLECroon
WARBLESing like a bird
WARBLESing quaveringly
WARBLESing, like a canary