The ancient ceremony dates back to when swans were a banquet delicacy

The Queen's swan marker and swan uppers took to the Thames in several small wooden boats, known as skiffs, to begin counting this year's cygnets.

The 12th Century tradition of weighing the young birds dates back to when swans were a banqueting delicacy.

Dressed in scarlet uniforms, the group set off from Surrey to head through Berkshire, Bucks and Oxfordshire.

The Crown owns all unmarked mute swans in open water, a right shared with the Vintners' and Dyers' livery companies of the City of London since the 15th Century.

Representatives from the two guilds will join Swan Marker David Barber on the river journey, checking the birds for injuries along the way.

The Queen's swan warden, Professor Christopher Perrins of the University of Oxford, will also ring each cygnet found with a unique identification number.

Decline in numbers

The ceremony enables experts to assess the risks posed to the creatures from vandalism, pollution, shooting and attacks from dogs and mink.

Preparing for the swan count last month, Mr Barber predicted shootings and vandalism will have caused a decline in numbers.

"Mindless shootings of swans and cygnets, often resulting in fatalities, threaten swan numbers and can have devastating consequences," he said.

"Furthermore, birds sitting on nests have been attacked with stones, with the result that eggs and nests are frequently destroyed."