Sound made by a bird (7)
(Other definitions for whistle that I've seen before include "Make a high-pitched noise by blowing air" , "Pucker up and blow" , "Make nonchalant noise" , "For something, ask in vain" , "Instrument; suit (slang)" .)
'sound made by a bird' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for chirrup that I've seen before include "Bird's noise" , "Chirp" , "Sound made by a bird" , "Tweet" , "Trill" .)