LA GOMERA — Sitting atop a cliff in the Canary Islands, Mr Antonio Márquez Navarro issued an invitation — “Come over here, we’re going to slaughter the pig”— without speaking a word: He whistled it.

In the distance, three visiting hikers stopped dead in their tracks at the piercing sound and its echo bouncing off the walls of the ravine that separated them.

Mr Márquez, 71, said that in his youth, when local shepherds rather than tourists walked the steep and rugged footpaths of his island, his news would have been greeted right away by a responding whistle, loud and clear.

But his message was lost on these hikers, and they soon resumed their trek on La Gomera, one of the Canary Islands, a volcanic archipelago in the Atlantic that is part of Spain.

Mr Márquez is a proud guardian of La Gomera’s whistling language, which he called “the poetry of my island.” And, he added, “like poetry, whistling does not need to be useful in order to be special and beautiful.”

The whistling of the Indigenous people of La Gomera is mentioned in the 15th-century accounts of the explorers who paved the way for the Spanish conquest of the island.

Over the centuries, the practice was adapted to communicating in Castilian Spanish.

The language, officially known as Silbo Gomero, substitutes whistled sounds that vary by pitch and length for written letters.

Unfortunately, there are fewer whistles than there are letters in the Spanish alphabet, so a sound can have multiple meanings, causing misunderstandings.

The sounds made for a few Spanish words are the same — like “sí” (yes) or “ti” (you) — as are those for some longer words that sound similar in spoken Spanish, like “gallina” or “ballena” (hen or whale).

“As part of a sentence, this animal reference is clear, but not if whistled on its own,” said Ms Estefanía Mendoza, a teacher of the language.

In 2009, the island’s language, officially known as Silbo Gomero, was added by the United Nations Educational, Scientific and Cultural Organization to its list of the Intangible Cultural Heritage of Humanity; the United Nations agency described it as “the only whistled language in the world that is fully developed and practised by a large community,” in reference to La Gomera’s 22,000 inhabitants.

But with whistling no longer essential for communication, Silbo’s survival mostly relies on a 1999 law that made teaching it an obligatory part of La Gomera’s school curriculum.

On a recent morning at a school in the port town of Santiago, a classroom of 6-year-olds had little difficulty identifying the whistling sounds corresponding to different colours, or the days of the week.

Things got trickier when the words were incorporated into full sentences, like “What is the name of the child with the blue shoes?” A couple of the children argued that they had instead heard the whistling sound for “yellow.”

If interpreting a whistle isn’t always easy, making the correct sounds can be even harder. Most whistlers insert one bent knuckle into the mouth, but some use instead the tip of one or two fingers, while a few use a finger from each hand.

“The only rule is to find whichever finger makes it easier to whistle, and sometimes unfortunately nothing works at all,” said Mr Francisco Correa, the coordinator of La Gomera’s school whistling program.

“There are even some older people who have understood Silbo perfectly since childhood, but never got any clear sound to come out of their mouth.”

Two whistlers might struggle to understand each other, particularly during their first encounters — and need to ask each other to repeat sentences — like strangers who speak the same language with different accents.

But “after whistling together for a while, their communication becomes as easy as if speaking Spanish,” Mr Correa said.

As is the case in many languages, whether whistled or not, there is a generation gap on La Gomera.

Mr Ciro Mesa Niebla, a 46-year-old farmer, said he struggled to whistle with a younger generation trained at school because, he said, “I’m a mountain guy who learned at home to whistle the words our family used to farm, but I don’t have the vocabulary of these kids who learn salon whistling, which is a bit too fancy for me.”

Some older residents have also stopped whistling because of tooth problems. Mr Márquez continues to whistle with his dentures, “but it’s not as easy and as loud as when I could press my finger onto my real teeth,” he said.

With its distinct geography, it’s easy to see why whistling came into use on the Canaries; on most of the islands, deep ravines run from high peaks and plateaus down to the ocean, and plenty of time and effort are required to travel even a short distance overland.

Whistling developed as a good alternative way to deliver a message, with its sound carrying farther than shouting — as much as two miles across some canyons and with favourable wind conditions.

Older residents on La Gomera recall how Silbo was used as a warning language, particularly when a police patrol was spotted searching for contraband.

In a recent fictional movie, The Whistlers, Silbo is used by gangsters as their secret code language.

Some other islands in the archipelago have their own whistling languages, but their use has faded, though another island, El Hierro, recently began teaching its version.

“Silbo was not invented on La Gomera, but it is the island where it was best preserved,” said David Díaz Reyes, an ethnomusicologist.

Nowadays, La Gomera relies heavily on tourism, which has created an opportunity for some young whistlers like Lucía Darias Herrera, 16, who has a weekly whistling show at an island hotel.

While she normally whistles Castilian Spanish, she can also adapt her Silbo to other languages spoken by her audience, on an island that is particularly popular with Germans.

Since last spring, however, the coronavirus has not only cancelled such shows, but also forced schools to limit their whistling instruction. At a time of compulsory face masks, a teacher cannot help a student reposition a finger inside her mouth in order to whistle better.

Younger children also “make huge efforts to blow out a lot of air, which means some are spitting rather than whistling,” said Mr Correa, the school coordinator.

So as a precaution against spreading the virus, the children now spend their weekly whistling lesson listening to recordings of Silbo, rather than whistling themselves.

An added difficulty for the students is that they don’t always have much opportunity to practice Silbo outside school. In the class of six-year-olds, only five of 17 raised their hands when asked if they had a chance to whistle at home.

“My brother actually can whistle really loudly, but he won’t show me, because he is either on his PlayStation or out with friends,” complained one of the youngsters, Laura Mesa Mendoza.

Still, some teenagers enjoy whistling greetings to each other when they meet in town and welcome the chance to chat without many of the adults around them understanding.

Some had parents who went to school before learning Silbo became mandatory, or who settled on the island as adults.

However much she is attached to her cellphone, Erin Gerhards, 15, sounded keen to improve her whistling and help safeguard the traditions of her island.

“It is a way to honour the people that lived here in the past,” she said. “And to remember where everything came from, that we didn’t start with technology, but from simple beginnings.” THE NEW YORK TIMES