The WhatsApp groups have one simple rule: typing and speaking are forbidden, on pain of immediate removal.

Only voice notes with whistling are allowed, although the choice of content is up to the sender: it can be an imitation of a bird or a tune such as the theme from The Pink Panther or the introduction to Scorpions’ Wind of Change.

In recent days, Brazil has seen a sudden craze for whistling-only WhatsApp groups, bringing together people of different ages and professions, including members of Congress.

Some of the groups get up to 600 voice notes a day, with the most popular performances racking up hundreds of thousands of views on TikTok, such as interpretations of Evanescence’s Bring Me to Life or Gorillaz’s Feel Good Inc.

New groups are emerging every day in the world’s second-largest WhatsApp market, some reaching the 1,024-member limit in less than 24 hours, prompting administrators to create a new one and repeat the cycle.

The results are often unimpressive, and judgment falls to emojis and stickers – the only short-text format allowed.

A good whistle can earn a sticker echoing the “absolute cinema” meme – but with Martin Scorsese’s face replaced by a toucan or parakeet and the phrase “absolute assobio”, the Portuguese word for whistling. If it is bad, the most common criticism is that there is more wind than melody.

Some members put in effort, playing a karaoke version of a song in the background and whistling the vocal part, while others simply let out one or two chirps as they wash the dishes.

“I saw a video online about some groups that already existed and we decided to create one with a few friends just for fun,” said Enzo Dias, a business administration student, who created one group.

“It was meant to be just for us; one friend invited another and before we knew it there were more than 500 people. The next day, we reached maximum capacity and had to create another,” said the 18-year-old, who set up the first group on 13 April.

No one knows exactly how the trend started, but the earliest TikTok posts date back to mid-April. On Google Trends, searches for whistling in Brazil reached their highest level on record in April.

“It’s so many messages that we have to close the group overnight, because we’re not able to check if someone is breaking the rules,” said Dias about the strict ban on typing and speaking.

To avoid non-whistling loiterers, some groups also require at least one whistle a day. Most members are men, and some women-only groups are now emerging.

The vast majority of groups are free to join and shared on social media – and some are warning about the digital security risks of joining groups with strangers via open links.

“I think most people are in it just for fun, but there are some who seem almost addicted to whistling and spend the entire day doing it,” said Dias. His group organises competitions through its Instagram page, where followers can vote on voice note duels.

The craze has spawned other highly niche groups in which participants are only allowed to imitate dogs, cats or donkeys, or share photos of cars in a single specific colour.

Dr Adriana Amaral, the coordinator of the CULTPOP research laboratory at the Fluminense Federal University in Rio de Janeiro, saw the trend as a repetition of others that have existed since the popularisation of the internet, such as the “I hate Mondays” groups on Orkut, or flashmob movements.

“Historically, these ephemeral forms of aggregation have always existed and are linked to the way pop culture itself operates: they allow people to channel certain rituals, habits and tastes into communities of affinity,” she said.

Ayran Ferreira, 18, a law student who also runs one of the groups, does not believe he will be hearing hundreds of whistles a day much longer.

“People want to take part just because it’s trending online, and they’re creating groups for all sorts of things – I’ve even seen one for imitating the sound of a backhoe [a type of excavator] – but I think it will eventually fade,” he said.