Xavi Bou spends most weekends photographing birds. Although he appreciates their plumage and mating habits and everything else nature photographers love about the animals, he isn't especially interested such things. He's far more interested in the hypnotic patterns birds create while flying.

Bou snaps hundreds of photos of birds in flight and stitches them together in Photoshop, compressing several seconds of movement into one frame. The photos in *Ornitographies *are as stunning as they are haunting. "[It] shows the hidden beauty of nature,” Bou says.

Bou started birdwatching in the 1980s with his grandfather, who took him to the Llobregat Delta wetlands of Catalonia. He now lives in Barcelona, and often visits reserves like the Lleida drylands and the Ebro Delta. While exploring the Empordá wetlands in 2011, Bou wondered what it might look like if birds left tracks as they flew. "I started to look for a technique that helped me to freeze some seconds of flight in a single picture," he says.

He tried making long exposures showing the bird's trajectory, but found the images too blurry. Bou realized he needed to combine multiple shots, a technique that draws on the chronophotography that Étienne-Jules Marey and Eadweard Muybridge pioneered in the 19th century. It worked exactly as he envisioned.

Bou uses a variety of cameras, including a Sony F27 and Blackmagic Ursa Mini 4K, and shoots 30 to 60 frames per second. A final image might include more than 600 shots woven together into amazing patterns—the black slash of starlings, the lazy loops of storks, the frenetic lines of swifts. The shapes are as varied and beautiful as the birds that created them.