The number of cygnets had actually fallen by 18% down-river

David Barber, Queen's Swan Marker

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.

Ownership of the swans is shared with the Vintners' and Dyers' Companies who were both granted rights by the Crown in the late-15th century.

Of the 132 cygnets, the Queen retained ownership of 70 and, the Worshipful Vintners' Company owned 30 and the Worshipful Dyers' Company 32.

Mr Barber said that conditions for the swans to breed seemed to be better up-river.

The Swan Marker counts the cygnets each year

He said: "There is more vandalism and fishing down-river between Sunbury and Marlow, and the natural habitat is not so good.

"The number of cygnets had actually fallen by 18% down-river."

It has always been the duty of the Sovereign's Swan Marker to count the young cygnets each year and to ensure that the swan population is maintained.

The Swan Warden, Professor Christopher Perrins, of Oxford University, also gives the swans a health check.