The tradition that the swans of Britain belong to the Queen (or, more correctly, the reigning monarch) is a commonplace 'pub fact' that actually has some basis in truth. As ever, the truth is slightly more nuanced than the soundbite.
Like most wildfowl in the early medieval period, swans often found themselves on the dinner tables of rich and poor alike. Prior to the arrival of domesticated poultry from Asia in later centuries, the swan was a large and readily available source of meat - although it was the young cygnets who were said to have the greatest flavour and were served at feasts.
Of course in those straitened times possession of such birds came to signify wealth and power and, like deer, became the prerogative of the aristocracy. The earliest written record of royal ownership of swans dates back to 1186 and actually relates to captive birds - indicated that swans were already being farmed to some degree.
The various grand houses and families of England laid claim to swans on the waterways within their bounds and from the 13th century onwards would mark their birds with elaborate carvings in their beaks that signified their ownership. These marks were made on the living birds during 'swan upping' season - organised by the crown around July while the cygnets were still unable to fly. This was a labour-intensive task, involving boats and man power to ply the waters, catch the birds and mark them. Poaching and eating marked swans was met with severe punishment.
The crown was therefore merely one owner of swans - those within royal lands - with the rest owned or claimed by the various noble houses.
As farmed poultry became more commonplace, swans played an increasingly marginal role at the dinner table and by the 19th century very few houses bothered to retain their rights to swans.
Today, only the crown (in conjunction with the Vintners and Dyers) retain their right to swans on the Thames only - and that in a purely ceremonial fashion with an added dash of modern ecological concerns. Swan Upping still takes place during a colourful five days in July, and the Queen's keeper of swans still oversees a count of all swans. However, the swans are not served as a tasty titbit, but counted as part of a general census of wildlife and monitored for health problems.