A team of official Royal Swan Uppers in Henley will be checking the numbers of the swan population today as part of a historic annual ceremony called 'swan upping'.
A swan census is carried out on the stretches of water throughout Surrey, Buckinghamshire, Berkshire and Oxfordshire.
The ceremony dates back from when the Crown owned all mute swans and they were checked to ensure people weren't poaching the animals to eat at banquets and feasts.
In the historic annual event, the Queen's Swan Marker, the Royal Swan Uppers and the Swan Uppers use six traditional rowing boats during their 5-day journey.
They will wear traditional red uniforms and each boat flies royal flags.
When a brood of cygnets is sighted, the men shout "All up!", where other boats should get into position.
Also, on passing Windsor Castle, the rowers must stand to attention in their boat with oars raised and salute "Her Majesty The Queen, Seigneur of the Swans".