If you ever visit La Gomera, one of the small islands that make up the Canaries, you are guaranteed to hear some sweet nightingale like whistling sounds. While some may emanate from the golden-voiced bird itself, chances are most of what you are hearing is coming from the residents, who believe it or not, are tweeting actual words to each other in a language they call Silbo Gomera (Spanish for 'Gomeran Whistle') or El Silbo (the whistle).
Nobody knows for sure how this whistling language originated, since it was apparently invented even before the original residents of the island, the now extinct race known as the Gaunches came to settle there. It is believed that before the last of them left the island in the 16th Century, they passed it on to the Spanish settlers who not only adopted it, but also, translated it to Spanish.
As is the case with most ancient languages, it flourished until about the 19th Century and then began to die down. By the late 20th Century, barely anyone could 'speak' this unique whistling language. Alarmed at the loss of what was regarded as a cultural heritage, the local government decided to step in and make the language mandatory for all students.
Today, all the residents of this tiny island are proficient in Silbo Gomera and take great pride in chirping away at each other. Also, while there are a few other whistling languages around the world this, is the only one that is fully developed - One where every vowel and consonant of the local spoken language, Castilian Spanish can be mimicked by distinct whistles, making for some real sweet sounds!
Resources: odditycentral.com, wikipedia.org