Every year, millions of monarch butterflies make a journey like no other, migrating 4,000km from Canada to Mexico to escape the harsh North America winter. It was only in 1976 that scientists recognised this remarkable natural phenomenon, but today the butterflies are under threat, their habitats destroyed by illegal logging and intensive agriculture. "It's incredible," says Canon Ambassador Pascal Maitre, who spent a week in January 2019 photographing the creatures. "The butterflies normally live for five weeks, but as the summer ends they produce a new generation that can live for eight months, so they can travel all the way to Mexico and back."

Pascal's shoot began life as a commission for the Yves Rocher Foundation, which runs reforestation programmes and butterfly sanctuaries in Michoacán, Mexico, where the butterflies land. "It wasn't a typical corporate story. They wanted me to show not just what they do but the benefits of what they do," says Pascal of the project, which was intended for exhibition at Festival Photo La Gacilly in Brittany, France, but this was postponed twice because of Covid-19 restrictions. Before departing for the shoot, Pascal mentioned the commission to Le Figaro Magazine, who sent a writer along for a few days of his trip. The feature was finally published in the magazine in June 2021. This was partly a consequence of the exhibition delay, but it's also common in magazine publishing. "It often happens that there's a long time between shooting and publication," says Pascal. "With National Geographic, for example, it can be one or two years."

To get to Michoacán entailed a flight from France, where Pascal is based, into Mexico City, then driving 250km and trekking for two hours in the mountains. As usual, Pascal worked alone, with no photography assistants, just a local driver, who also acted as a guide. "That's why I like photography – you can do it on your own," he says. He focused on three sanctuaries – one larger one, open to the public, and two smaller ones where scientists work. "It depended on the sun or the wind how many butterflies would move at the same time. When you do a story, it's always like this. Some days there is good light, some days there isn't. You have to work and work and never be satisfied because maybe on the last day, you'll get the best picture."

Despite his vast experience, Pascal found that the project posed unique challenges. "This is the most difficult story I've ever done in terms of photography," he says. "The butterflies are small and they move so fast. With video that would be easy but with still photography it's a real challenge to freeze the action and keep the focus. You can't use a strobe because this disturbs the butterflies and they lose their way."

His solution was to "shoot blind", focusing about two or three metres away and at times setting his aperture as narrow as f/22 to ensure an extensive depth of field, removing the need to find individual focal points within a mass of fluttering butterflies. Simultaneously, he cranked up his shutter speed as far as 1/4000 sec when needed, to freeze the movement of the butterflies, while pushing sensitivity as high as ISO5000 to maintain exposure. He took "thousands and thousands and thousands of pictures", which made for a laborious edit but ultimately paid off handsomely.

Although this wasn't technically a wildlife commission, understanding butterfly patterns of behaviour served Pascal well. He would make sure he was in the right spot at the right time, observing how the butterflies would congregate and travel around, flying en-masse to a river during the day in search of water or bedding in for the night in the treetops as darkness fell.

One of the images that stayed with him was the way the butterflies look in the early hours. "People tend to show the butterflies' orange wings but the back of their wings is black. This is something you only see at night. This is what's difficult in reportage, convincing your driver to wake up at 2:30 in the morning when the first light is coming so you can see what's happening. But I think that's what makes this different from other stories you see about these butterflies."

Even when you're an experienced photography veteran like Pascal, each assignment teaches you something new. This one was a lesson in pushing the technical possibilities of his kit. He compares his role as a photojournalist piecing together a story to that of a detective on an investigation. "When I shoot, I'm very free, I will try anything. You have a problem and you have to find a solution, technical or editorial. This is our job."

But, he adds, bearing witness to this breathtaking spectacle had a lasting impact on him, personally as well as professionally. "Each day, I was excited. To shoot this wasn't easy but to see it was incredible – really magic."