Scarlet-clad emissaries of Britain's Queen Elizabeth II began paddling southern England's sprawling River Thames this week in search of swans for the ancient tradition of "swan upping."

'Swan Upping' is the annual census of the swan population on stretches of the River Thames. [Xinhua]

The annual royal census of swans on the Thames, which dates from the 12th century when the English crown claimed ownership of all mute swans and the birds were considered a delicious dish, takes place during the third week of July every year.

Her Majesty's Swan Marker and his team of Swan Uppers, identifiable by their scarlet uniforms, weigh and measure cygnets to determine growth rates. The Swan Warden checks the swans for signs of disease and injury before ringing the birds with individual identification numbers and releasing them.

A procession of six traditional rowing skiffs travel stretches of the Thames over five days, passing through the southern English counties of Surrey, Buckinghamshire, Berkshire and Oxfordshire. Schools from local areas along the river are invited to watch each year.

"The Swan Marker and Swan Uppers will be meeting several schools during their five-day journey up river during which we talk to the children about The queen's ownership of mute swans and the history associated with it. We also discuss the mute swan's habitat and life cycle in a continuing attempt to raise their awareness of, and interest in, wildlife," Swan Marker David Barber said.

The Swan Upping report, compiled at the end of the census, provides data on the number of swans accounted for, including broods and cygnets. This information is used for the maintenance of the population.

The presence of disease in the swans will be monitored particularly closely this year after an outbreak of duck virus enteritis, a virus which has affected the mute swan population of the Thames. Fewer cygnets are expected on the river this year as a result, Barber said.

"Last winter more than 180 swans were found either dying or dead on the river between Reading and Windsor, with over 115 deaths being reported in the Windsor area alone," Barber said.

"The virus is not uncommon in swans and it has also affected swans in other parts of the country, but this was the worst outbreak on the Thames we have seen in many years."

Today the monarch retains the right to ownership of all unmarked swans in open water, although in practice this privilege is only exercised by the queen on certain stretches of the Thames.