Though the swans gracefully floating along the River Thames through Windsor, England may appear wild and free, they actually fall under the ownership of three distinct, dignified entities.

Queen Elizabeth II, inheritor of the ancient title Seigneur of the Swans, can lay claim to any of the unmarked mute swans within the open waters of England and Wales. But on the River Thames, she shares her stock with two medieval livery companies, the Worshipful Company of Vintners and the Worshipful Company of Dyers.

In the 12th century, the Crown declared itself the owner of any and all mute swans within England to protect its supply of what was then considered a delectable delicacy. However, in the 15th century, the companies Vintners and Dyers were given rights to the beautiful white birds on the Thames. To claim their swans, the two companies would carve distinct marks into their beaks.

Though technically still in effect across England and Wales, the Queen’s all-encompassing mastery of the unmarked mute swans is really only observed on some stretches of the Thames, particularly the area around Windsor. The birds are counted and claimed in the annual Swan Upping census, a tradition that’s now roughly 900 years old.

Each July, three teams of Swan Uppers representing each of the swan-owning entities board their traditional rowing skiffs and embark upon a five-day mission to count the birds. They use their boats to surround the swans, then scoop both the adults and cygnets from the water to weigh, measure, and tag them. The two livery companies still mark any fowl they wish to claim, though they do it with leg bands rather than carvings.

Now, the long-running tradition is more about conservation than preserving a potential food source. Swans are no longer commonly consumed, though one of the Queen’s birds was illegally barbecued and eaten in 2013. Others have also been shot by vandals.

While rowing by Windsor Castle, it’s tradition for the Uppers to toast to “Her Majesty the Queen, Seigneur of the Swans.” The Queen isn’t usually home to appreciate the gesture—she’s only attended one Swan Upping throughout her entire reign.