The 900-year-old survey is expected to show a further decline

The annual census of the swan population on stretches of the river is expected to confirm the fall.

The impact of mink attacks - blamed for a decline last year - is not expected to be as "devastating" this year.

The predictions came from the Queen's Swan Marker as he announced the dates for this year's Swan Upping, the ancient survey of the Crown's swans.

Mr Barber says swans suffer at the hands of vandals

David Barber, who will carry out the census with the assistance of swan warden Professor Christopher Perrins, of the University of Oxford, said: "We have seen a decline in the number of breeding pairs nesting along the central River Thames area this year.

"As with previous years, shooting, vandalism and dog attacks have also contributed to a decrease in numbers.

"The impact of mink attack has not been as devastating as last year when they contributed to a high proportion of overall fatalities.

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

Mute swans

Children from local schools are invited to join the uppers this year to learn about swan conservation.

The Upping will start on 19 July at Sunbury-on-Thames, Surrey, and will end at Abingdon, Oxfordshire, on 23 July.

The census dates from the 12th Century when the Crown claimed ownership of all mute swans.

Nowadays, the Crown retains the right to ownership on some stretches of the River Thames and tributaries.

Ownership is shared with the Vinters' and Dyers' Companies which were both granted rights by the Crown in the 15th Century.