The survey dates back to the 12th century

The official who carries out Swan Upping, the traditional count of cygnets, said there had been "many" attacks by vandals.

David Barber, the Queen's Swan Marker, said: "The majority of incidents involve the destruction of nests and eggs."

The survey, which dates back to the 12th century, is carried out in the third week in July using traditional rowing skiffs.

Mr Barber said: "There has been a slight decrease in the number of breeding pairs nesting this year, although the size of the broods have been high.

Humans to blame

"There have been many reported fatalities, a high proportion of which have been the result of mink attacks.

"Mink are on the increase throughout the Thames area and young cygnets are extremely vulnerable to attack."

But Mr Barber said humans were still the biggest factor in the decline in swan numbers.

"Many reports have been received of vandalism this year," he said.

"Fishing hooks and line are also a significant factor in the injuries and deaths of many young cygnets.

Health check

"Discarded fishing tackle poses a severe threat of drowning and ingested hooks and line cause immense suffering which often results in death."

With the assistance of the Swan Warden, Professor Christopher Perrins of the University of Oxford, the swans are also given a health check.

The Swan Marker is also accompanied by the Swan Uppers of the Vinters' and Dyers' livery companies and uses six Thames rowing skiffs decked out with traditional pennants and flags.