Also, Art's posting reminded me of this, which I thought might be of interest:

Canary island whistles again
        La Gomera
Silbo developed as a means to talk across La Gomera's great ravines


A means of communication using whistling is being revived after nearly vanishing from the one island on which it is used.

The language is called Silbo Gomero, and is only heard on the Canary Island of La Gomera, off the coast of Morocco.

Until recently those who communicated in Silbo were dying out — but the government of the island made it compulsory for all schoolchildren on the island to study it, and now it is making a comeback.

"There are real masters of Silbo, but most of them are now very old," Francisco Rivero, a researcher in the University of La Laguna in the Canary Islands, told BBC World Service's Outlook programme.

"So the local government decided to introduce it to elementary schools, so that children can learn the Silbo technique.

"It's taught in schools as a way of making children aware of their local culture."

Berber link

Silbo has only four vowels and four consonants.   The key to it is understanding the meaning of the many different tones of the whistles.

It can be heard more than two miles away — which was the key to its being sustained on the La Gomera.
        
It's practically a language in itself — just like Castilian Spanish — but it relies on tones rather than vowels and consonants.

The language has been passed on from father to son as it was essential to be able to communicate over long distances across the inaccessible valleys.

"The island is very hilly, with lots of ravines, which make communication very difficult," explained Dr Rivero.

As a result, a tradition developed whereby if one person heard a whistle, they passed it on.

Islanders got so skilled at it that messages have been successfully passed right from one end of the island to the other.

"Historically, from the earliest settlers on the Canaries, the Silbo language was the mobile phone of the period," Dr Rivero said.

"[It] allowed people to communicate across great distances, because its frequency allowed the sound to be transmitted.

"This form of communication dates back before the Spanish conquest, in the 15th Century."

Structure

Silbo is believed to have come to the island from the Berber people of Morocco, Dr Rivero added.
        
The Canary Islands have very strong links with Morocco, particularly the Berbers, and there is evidence that there may be some people deep in the Atlas mountains who also use whistling to communicate.

However, Silbo on La Gomera is unique as it has adopted Spanish speech patterns.

"It's practically a language in itself — just like Castilian Spanish — but it relies on tones rather than vowels and consonants," Dr Rivero stated.

"The tones are whispered at different frequencies, using Spanish grammar.   If we spoke English here, we'd use an English structure for whistling.

"It's not just disjointed words — it flows, and you can have a proper conversation."