I remember coming across this once, many years ago. An unusual whistle language is used by shepherds on one of the Canary Islands off the northwest coast of Africa. [Via Shepherds whistle while they work and brains process sounds as language.]
The language studied by Corina and his colleague, Manuel Carreiras, a psychology professor at the University of La Laguna, on the island of Tenerife in the Canaries, is Silbo Gomero, or Silbo. It is primarily used by shepherds to communicate with each other over long distances of rugged terrain on the island of La Gomera, another island in the Spanish owned Canaries.
… Silbo is believed to have been brought to the island by Berbers from North Africa and today is a surrogate language for Spanish. It condenses Spanish into two vowels and four consonants. … Silbo is an occupation-centered language and is used to say such things as “open the gate” or “there is a stray sheep.” It is not the world’s only whistle language. There are others in Greece, Turkey, China and Mexico, according to Corina.
Language Miniatures 3, Whistled languages, sheds a bit more light on this:
The Indian language … Mazateco, [is] spoken in southern Mexico near Oaxaca. Its speakers live in a mountainous region, and while most of their everyday talking is through the normal vowel-consonant channel, when two people need to say something across a wide valley, whistling what they have to say is a very efficient and effective channel. Since the rhythms of village life tend to be fairly predictable, the speakers of this language are obviously being aided by their mutual knowledge of what is likely to be said.