By prerogative right, the British Crown enjoys ownership of all unmarked mute swans in open water. Rights over swans may, however, be granted to a subject by the Crown (accordingly they may also be claimed by prescription.) The ownership of swans in a given body of water was commonly granted to landowners up to the 16th century. The only bodies still to exercise such rights are two livery companies of the City of London. Thus the ownership of swans in the Thames is shared equally among the Crown, the Vintners' Company and the Dyers' Company.
Swan upping is the traditional means by which the swans on the Thames are apportioned among the three proprietors. Its main practical purposes today are to conduct a census of swans and check their health. It occurs annually in the third week of July. Over five days, the Queen's, Vintners' and the Dyers' respective swan uppers row up the river in skiffs in recent centuries from Sunbury-on-Thames to Abingdon on Thames. Swans caught by the Queen's swan uppers under the direction of the Swan Marker are left unmarked, except for a lightweight ring linked to the database of the British Trust for Ornithology (BTO). Those caught by the Dyers and Vintners receive a similar ring on the other leg. Originally, rather than being ringed, swans' bills would be nicked using a metal implement, a practice reflected in the pub name The Swan with Two Necks in the City connected with the Vintners, a corruption of "The Swan with Two Nicks".
On 20 July 2009, Queen Elizabeth II, as "Seigneur of the Swans," attended the Swan Upping ceremony for the first time in her reign. This was the first time that the monarch had personally watched the ceremony in centuries.
In 2012, exceptional high river flows for summer prompted a partial cancellation: between Sunbury-on-Thames and Windsor, the first definitively known cancellation (albeit partial) in its 900-year history.
The first known full cancellation took place in 2020 due to Covid-19 social distancing measures. 
- The Case of Swans (1592) 7 Co. Rep. 15b
- Wheals, Brian Brenchley (1984). Theirs Were But Human Hearts: A Local History of Three Thameside Parishes. H.S. Publishing. p. 65. ISBN 0-9509053-0-5.
- The Queen's Swan Marker (2018), Royal Swan Upping, Trinityweb Ltd
- "Windsor Swan Upping ceremony cancelled due to flooding". BBC News.
- "Royal tradition of swan upping cancelled this year due to coronavirus". The Telegraph.
- Norman Frederic Ticehurst, The Mute Swan in England: Its History, and the Ancient Custom of Swan Keeping (1957).
Media related to Swan Upping at Wikimedia Commons