Film shot this weekend at the Nonsuch Island sanctuary, where another year’s cahow chicks are busy fattening up in preparation for their eventual life out on the ocean, will feature in an upcoming documentary exploring the distant travels of one of the world’s rarest seabirds.

Deirdre Brennan, creator of the 2004 filmBermuda’s Treasure Island, is back to follow up for an international audience on the success story of the bird that almost wasn’t — the Bermuda petrel, thought exterminated for centuries until its rediscovery 75 years ago.

The Royal Gazette joined researchers at the Castle Harbour nature reserve as the camera followed conservation officer Jeremy Madeiros’s inspection of the chicks’ progress.

“Oh boy, he’s grown since the last time I took a look — and he feels like he was fed last night,” said Mr Madeiros, lifting a cheeping, down-covered chick from its burrow for a weight check while measuring its developing wings.

Under three weeks old, the cahow chick was already keenly flapping.

Until recently, exactly where the birds went after zooming from Nonsuch to the open ocean was largely a mystery. Scientists have found that cahows catch sea winds to travel not hundreds but thousands of miles around the Atlantic Ocean.

Ms Brennan said of her latest film project, under the working title Secret of Devil’s Isle: “There are so few conservation success stories. A story like this — everyone should know about it.”

Seabird researcher Letizia Campioni, at present contracted to the Biological Station of Doñana in Seville, Spain, explained: “We want to understand where they go, what happens when the bird leaves the nest until it returns for the first time.”

Fledgelings fitted with geolocator trackers in 2022 when they set out to sea are now homing back in on Nonsuch as adult birds, with the devices yielding valuable details on their journeys in between.

Fellow researcher Francesco Ventura, who works at Woods Hole Oceanographic Institution in Massachusetts, said: “By looking at these recordings, we will know when they are flying and when they are soaring. We will be able to understand how they do what they do.”

Cahows, Dr Ventura added, are “creatures of the wind — they surf the wind, and I am studying how they do it”.

The tags on the birds not only show where they forage but reveal their flight strategies.

Cahows travel long distances to snatch deep-sea life surfacing at night that they can spot capably in the dark.

The birds have no problem sleeping while aloft. Since they only come to land for breeding, cahows drink seawater, filtering and sneezing the salt back out.

They are also “amazing flyers”, Dr Campioni said.

Beyond letting it carry them, cahows make selective use of the wind, employing specialised wing mechanisms to catch wind shear close to the ocean with a minimum expenditure of energy.

The impact of climate change is also on the scientists’ radar, Dr Campioni said.

“The cahow is associated with the Gulf Stream. We’re interested to see if any changes in the ecosystem will affect the behaviour and movement of the birds.”

Their work partners with Mr Madeiros and Jean-Pierre Rouja, of Nonsuch Expeditions, whose recordings of the birds, including what goes on in their secretive burrows, have captivated audiences in Bermuda and abroad.

James Doughty, the cameraman this weekend on Nonsuch Island for Ms Brennan’s documentary, said he was asked to join courtesy of his filming with conservation groups, safari lodges in Africa and Burnt House Productions in Bermuda.

“Growing up in Bermuda, I was lucky enough to come out here,” Mr Doughty said.

“We’re still in the early days of filming. We’ve done about four filming days in total out of who knows how many.

“I’m excited to see how the chicks develop, and how our story develops.”

The project, tapping into research from five countries, has informed new scientific papers for a bird that started hatching naturally on Nonsuch Island in 2009, for its first time since the 1620s.

Because cahows spend so much of their lives at sea, the birds have little cause to fear humans.

It proved their downfall when Bermuda was settled and colonists needed an easy meal — particularly after the arrival of rats, which overwhelmed the island, devouring crops and pushing famished European settlers to desperation.

Although far less aggressive than longtails, the birds are not always docile. The backs of Mr Madeiros’s hands are peppered with scars to prove it.

However, the birds today largely remain as oblivious to the presence of man as they were centuries ago.

Mr Madeiros shared a story from a recent night on Nonsuch watching cahows at flight over their old home.

An arriving bird flapped in to land a short distance from the researcher, heading for the site where it had grown up. Mr Madeiros said their keen memories and sense of smell always guide them back.

“It came up to me, and it crawled up into my lap,” he said.

This weekend, taking a break from checking on cahow chicks, Mr Madeiros said: “It’s a really good team that’s been built up. We’re in a really good place now.”

According to Ms Brennan, the finished work is hoped to be screened early next year.