Whistlers

Kircher Society: "On the small mountainous island of La Gomera, one of the Canaries, the children speak to each other from miles apart using one of the most unusual languages in the world. Known as Silbo, the whistling language of Gomero Island has a vocabulary of over 4,000 words, and is used by 'Silbadors' to send messages across the island’s deep valleys. Though Silbo was on the verge of extinction in the 1990s, the Gomerans have made a concerted effort to revive their language by adding it to the public school curriculum. Today 3,000 schoolchildren are in the process of learning it."

There's a video about the island narrated totally in Silbo at the link there. A vocabulary of over 4000 words, all in whistles.