Swans on the River Thames are under threat from dog attacks and the American mink, the Queen's Swan Marker has said.
David Barber said the number of cygnets were down on last year as his team began an annual population survey.
Swan Upping dates to the 12th Century, when the Crown began counting numbers in order to ensure a supply for feasts.
Mr Barber said: "We've had a pretty rough time with...dog attacks, all sorts of things - like mink."
"They're not indigenous to this country and they're breeding like mad on the river here, and they do take a lot of young cygnets," he added.
The Queen's Swan Marker has the duty of counting the number of young cygnets each year on certain stretches of the River Thames and its tributaries.
Teams of surveyors board rowing boats, known as Thames Skiffs, to count the number of young swans while checking for signs of disease or injury.
American mink are thought to have escaped into the wild from farms decades ago.
Mr Barber said his team had been encouraging people to keep their dogs on a lead in nesting season, but added it had been "very difficult" for the cygnets.