This story appears in the January 2018 issue of National Geographic magazine.

If birds left tracks in the sky, what would they look like? For years Barcelona-based photographer Xavi Bou has been fascinated by this question. Just as a sinuous impression appears when a snake slides across sand, he imagined, so must a pattern form in the wake of a flying bird. But of course birds in flight leave no trace—at least none visible to the naked eye. Bou, now 38, spent the past five years trying to capture the elusive contours drawn by birds in motion, or, as he says, “to make visible the invisible.”

First he had to shed the role of mere observer. “Like a naturalist, I used to travel around the world looking at wildlife,” he says. He began exploring photographic techniques that would allow him to express his love of nature and show the beauty of birds in a way not seen before.

View Images Evoking an airborne serpent, western marsh harriers glide above trees where great cormorants perch. The wildlife-rich wetlands of Estany d’Ivars i Vila-Sana, Spain, were drained around 1950 for agricultural use and restored in 2005.

View Images “In winter the starlings gather in large groups,” says Bou, who took this photo in the agrarian village of Arbeca, Spain. “Starlings often prefer cities and agricultural areas, where their food abounds.”

View Images At a calm bay north of Spain’s Ebro River Delta on the Mediterranean coast, a flock of greater flamingos is silhouetted in the water, while European herring gulls fly overhead.

View Images Near the fishing village of Roses, Spain, seagulls form a dreamy tableau as they chase a boat for scraps.

Ultimately he chose to work with a video camera, from which he extracts high-resolution photographs. After he films the birds in motion, Bou selects a section of the footage and layers the individual frames into one image. He finds the process similar to developing film: He can’t tell in advance what the final result will be. There’s one magical second, he says, when the image—chimerical and surreal—begins to emerge.