"A Whistler, with the slim frame and outsize wings of adolescence, emerged from a fog-bank. The shrill notes of his lips carried far and keenly. Tolk, who as Chief Herald guided the education of these messenger-scouts, cocked his head and nodded. 'We guessed it very well,' he said calmly."
-Poul Anderson, The Man Who Counts IN Anderson, The Van Rijn Method (Riverdale, NY, 2009), pp. 337-515 AT VIII, p. 387.
Sheila and I attended a demonstration of the Canary Islands whistling language. See here. Someone hid three items, e.g., a hand bag, a camera, a scarf, in a large room full of people in sight of a whistler. A second whistler entered the room, found the items and returned them to their owners as instructed entirely in whistles by the first whistler