In the days after the Brexit vote last year, I became obsessed with an oil painting called “Swan Upping at Cookham” [below, Stanley Spencer, 1915-1919], which portrays a scene from an ancient and colorful English tradition. “Swan Upping” refers to the annual summer voyage of a flotilla of wooden skiffs that sets off from the town of Sunbury-on-Thames on a five-day journey to catch all the swans on the upper reaches of the River Thames. The crews check the parentage of young birds and place a mark on them to claim their ownership: Some belong to the queen, others to the Worshipful Company of Vintners and the Worshipful Company of Dyers, two ancient trade guilds based in the City of London. The painting depicts a traditional stop on the uppers’ trip. Here is the river and the Ferry Inn, wooden punts, moody clouds, women carrying cushions, a fretted iron bridge and a swan bound and hoisted in coils of rope and canvas, white neck craning from a man’s shoulder.
After some interesting background on the painting and on the place of these swans in English national mythology, Macdonald explains more about the legal historical background of the "upping":

