Adult: True to its name, Whistling Heron has high pitched vocalizations, variously described as "a high kee-kee-kee ca,, rapidly repeated, quite unlike the calls of other herons" (Friedmann and Smith 1950); as "a clear, loud, melodious whistle, about 1-1.5 sec long and often repeated in a series of 2-5 calls ca. 0.5 sec apart" (Kahl 1971); as a "high, throaty whistle of three long, identical notes" (Belton 1984); and as "a high, reedy, complaining whistle, often doubled or uttered in a ser[ies], wueeee, wueeee, ..." (Hilty 2003). This call is given both in flight and when perched; in calling, even in flight, the neck is extended and the bill held open (Kahl 1971, Belton 1984).
For a representative audio recording with sonogram, see audio
Young: Hisses (Kushlan and Hancock 2005).