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Swan Upping is the traditional Thames swan census dating back to the 12th century. Representatives of the Queen, plus the Vintners and the Dyers trade guilds, who historically claimed ownership of swans on the river, catch and mark as many of the seasons new cygnets as they can get hold of.
The original purpose of this was because swans were regarded as a delicacy at banquets, but these days Swan Upping is more of an ecological thing. The birds are weighed and checked for injury or disease and an ongoing census is kept.
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I have watched the Swan Uppers at work, and it's amazing how placid the swans are about the whole thing.