In the 1940s, during the early years of radar, systems operators regularly reported inexplicable blooms and blurs of light moving across their screens. They called them angels. During the Second World War these illuminated echoes even prompted an invasion scare in Britain; only later, in peacetime, were they identified as ordinary things and everyday occurrences – birds flocking in flight.

Starlings, gulls, swallows, kites, fulmars . . . many grey-black angels are to be seen for real in Xavi Bou’s photographs. Each of his Ornithographies shows actual birds undertaking actual journeys, and all the images here reveal wing work – the specifics of flying and how every species flies in its own way. No scientific photography or technical illustration known to me has rendered bird locomotion as legible. No artistic treatment of the same has been so literally built from the real world.

Most of these images record roughly twenty seconds of flying time. Each flight is rendered as a line, thin or thick and curved or straight depending on how the bird flies. The line is a capture of a flight path and is made from a thread of snapshots of the moving bird harvested from hundreds or more frames and collaged into one. Wingbeats are first stopped, as it were, and then reanimated. Every final print contains within it a sequence. The now (Cartier-Bresson’s ‘decisive moment’) of each is stretched by Bou, so that each has a past – earlier moments of the bird’s flight – within its present. The effect is similar to looking at a zoetrope or a flicker-book except everything happens in a single image. A whole movie is condensed into one photograph.

No bird could ever be seen by our naked eye as Bou shows it, but every flying bird actually moves in that way. Paradoxically, his artfulness increases each picture’s truthfulness.