A Buckingham Palace official said it was the first time the 83-year-old queen, who has reigned for 57 years, was on hand for the venerable practice of "Swan Upping." The annual census of the Thames' swan population dates to the time Britain's royalty jealously guarded its right to own and eat any mute swan - the species of graceful white birds distinguished by long necks and orange and black bills - in the river.
Although Britain has moved on from the days when the birds were prized as royal delicacies, the queen still theoretically has rights over all mute swans swimming on public rivers and lakes. Nowadays, the queen only ceremonially counts the swans on the River Thames.
Every year, a team of boatmen take traditional wooden skiffs flying the royal flag up the waterway to count how many of the graceful birds she owns.
The census helps monitor the health of the swan population. In the 1980s, swan uppers noticed a sharp decline in the population. The culprit was identified as lead fishing weights, which were poisoning the birds. They were banned, and numbers rebounded. This year's count is still going on but in recent years there have been 1,200 to 1,500 adult swans on the River Thames.
Clad in white trousers and red jerseys marked with royal insignia, the Swan Uppers - so-called because they pull the birds up out of the water - catch, measure, weigh, tag, and release them. The colorful exercise has more to do with conservation than cuisine these days. The weeklong census gives ornithologists a chance to monitor the health of the river's swan population and doubles as a lively environmental lesson for children and tourists who come to watch.
The monarch witnessed the ceremony from the restored 19th century steamer Alaska. Dressed in a coral-colored suit and wearing a swan-shaped brooch, she watched as boatmen handled the birds and smiled broadly as a she was shown an orphaned swan by her official Swan Marker, David Barber.
While the Swan Uppers always pay homage to the queen as they pass Windsor Castle, this is the first time the monarch has been present for the count. A spokesman for the queen said her engagement-packed schedule never allowed her time to see it - until now.
"It's one element of life in England she hadn't witnessed, so she decided to witness it herself," he said, speaking on condition of anonymity in line with palace policy.