The skiffs surround the swans so that they can be more easily caught.
Swan upping is an annual ceremony in England in which mute swans on the River Thames are rounded up, caught, ringed , and then released.
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…