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.)
[1] 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.