The first Swan Upping of the King’s reign is under way – with Charles’s Swan Marker and his team taking to the River Thames in newly updated uniforms bearing his cypher.

As King, Charles retains the right to claim ownership of any unmarked mute swan swimming in open waters, though this right is mainly exercised on certain stretches of the Thames.

Great British Life: It is the duty of the Sovereign's Swan Marker to count the number of young cygnets each year and ensure that the swan population is maintained.It is the duty of the Sovereign's Swan Marker to count the number of young cygnets each year and ensure that the swan population is maintained. (Image: Aaron Chown/PA Wire)

Great British Life: The annual census of the swan population on the River ThamesThe annual census of the swan population on the River Thames (Image: Aaron Chown/PA Wire)

Swan Upping is the annual census of the swan population and the ceremony is used to count the number of young cygnets, check their health and ensure numbers are maintained.

The King’s Swan Marker David Barber and his accompanying Swan Uppers of the Vintners’ and Dyers’ livery companies used six traditional rowing skiffs to begin their five-day journey upstream to Abingdon Bridge in Oxfordshire.

Great British Life: The King's Swan Marker, David Barber (left), with the Swan Uppers during the ancient tradition of Swan UppingThe King's Swan Marker, David Barber (left), with the Swan Uppers during the ancient tradition of Swan Upping (Image: Aaron Chown/PA Wire)

Charles’s flags were flying on the Crown boats and his CIIIR cypher was displayed on the arm of Mr Barber’s traditional scarlet jacket for the first time – as well as on the front of the red polo shirts worn by some of his assistants.

Mr Barber said the past year has been a “challenging” one for the swans because of the spread of bird flu, but although hundreds of the birds have died, reports of infection have fallen in recent months.

Great British Life: Swan Uppers during the ancient tradition of Swan UppingSwan Uppers during the ancient tradition of Swan Upping (Image: Aaron Chown/PA Wire)

Great British Life: The Swan Uppers capture birds before measuring them during the ancient tradition of Swan UppingThe Swan Uppers capture birds before measuring them during the ancient tradition of Swan Upping (Image: Aaron Chown/PA Wire)

Mr Barber was the late queen’s Swan Marker for 29 years and continues in the role for the King.

The ceremony dates back to the 12th century, when the ownership of all unmarked mute swans in Britain was claimed by the Crown in order to ensure a ready supply for feasts.

Abingdon is also noted for a 400-year-old tradition of bun-throwing; this event started for the coronation of King George III, but now takes place to celebrate significant national events. It most recently took place to mark the coronation of King Charles III.