Data on the swans is taken during the census

Mr Barber said: "Increased public vigilance along the Thames is helping to maintain swan numbers, and this year an above-average number of breeding pairs has been reported.

"Although swan attacks are down from the all-time high last year, a number of nests have still been vandalised and eggs have been either stolen or destroyed."

Data on the swans is recorded and at the end of the census the Swan Marker produces a report.

It provides information on the number of swans, weight of cygnets, estimated growth rates and any injuries suffered.

Census data

The Crown retains the right to ownership of all unmarked mute swans in open waters but the Queen only exercises her right on stretches of the Thames and its surrounding tributaries.

The birds are marked according to ownership during Swan Upping.

Swans with unmarked beaks belong to the Queen, those with a mark on the right belong to the Dyers' Company and swans with a mark on each side of the beak belong to the Vintners' Company.

While carrying out the Upping the Queen's Swan Marker and Uppers wear scarlet uniforms and their boats fly a royal flag.

As they row past Windsor Castle they stand to attention in the boats with their oars raised in salute to the Queen.

Mr Barber has been working with Swan Uppers from the Vintner and Dyer livery companies and the Queen's Swan Warden, Professor Christopher Perrins of Oxford University.