Her Majesty's cygnet service

For five days every year since the 12th century, servants of the Queen have taken to the Thames to carry out the ceremony of Swan Upping - the weighing and ringing of the royal swans

No, it's not a Butlins staff team-building exercise. This is Swan Upping, an ancient ceremony in which grown men spend a week dressed in bright scarlet to take the measure of the Queen's swans. This annual silliness dates from the 12th century, when the Crown claimed all unowned mute swans for banquets. Swan has long since fallen off the nation's menus but the ceremony of Swan Upping continues regardless.

On the Monday of the third week in July, small rowing boats - called skiffs - start out from Sunbury-on-Thames and make their way along the river. Along the route, swans and cygnets are weighed, ringed and checked for signs of illness. When the skiffs stream into Abingdon, in Oxfordshire, at 6pm today, it will mark the end of the ceremony, and the swans will be left to their own devices for another year.

The man in charge of Swan Upping is David Barber, the Queen's swan marker. Barber, who has held the year-round post for the past 12 years, wears a red coat with a jaunty feathered cap for his forays up-river. Under his command are dozens of "swan uppers", similarly ruddy volunteers from the Vintners' and Dyers' livery companies, who were granted rights to own swans by the Crown in the 15th century.

When Barber spies a group of swans, the cry of, "All up", is raised. Upon hearing the command, the swan uppers manoeuvre their vessels around the birds. Among much flapping of wings, the birds are settled, restrained and weighed. Beaks are then measured, an identification number attached, and the birds examined for fishing lines, vandalism or toxic chemicals. This swan-upmanship continues through Middlesex, Surrey, Buckinghamshire, Berkshire and Oxfordshire, except when the boats pass Windsor Castle, when all stand and say a toast to "Her Majesty, the Queen, Seigneur of the Swans".

"It's obviously a ceremonial thing," says Barber from the banks of the Thames, "but it does have a practical use, too. We are looking after the Queen's swans, and we are here to make sure those swans survive into the future. They're large, powerful birds - about 2.5m, and weighing about 15kg - so you have to know what you're doing to remove one from the water. I've had a few scrapes in my time - we all have. But we meet only once a year, so it's a great, happy, social event."