Fortunately, he has a powerful ally — Queen Elizabeth II.
One of the British monarch's many titles is Seigneur of the Swans — Lord of the Swans — and by ancient tradition she owns all mute swans found in Britain's open waters. In practice, she only exercises that right along the Thames, and every year she sends a flotilla of emissaries in rowing boats to count, measure and check the condition of her flock.
Known as swan upping, the five-day census is a uniquely British mix of ceremony and science that has been taking place since the 12th century.
Still, one major aspect of the census has changed.
"In those days, it was all about food," said David Barber, who wears a scarlet jacket and a white swan feather in his nautical cap and bears the glorious title of Queen's Swan Marker. "It was a very important food served at banquets at feasts. Today swan upping is all about conservation and education."
Barber has been leading swan upping expeditions for more than 20 years, accompanied by a zoologist and a score of boatmen clad in three liveries: red for the queen, white for the Worshipful Company of Vintners and blue for the Worshipful Company of Dyers. The last two are medieval London trade guilds granted ownership of some Thames swans in the 15th century.
Swans are now protected by law and no longer risk being turned into supper, but they still face many hazards, from mink and foxes to dogs, vandals and overhead power cables. Urban sprawl has also encased stretches of riverbank in concrete, endangering the birds' nesting sites.
"You could make (the river) more user-friendly for swans," said Chris Perrins, an Oxford University professor who holds the post of Her Majesty's Swan Warden. "I'd like to see more of an attempt to increase the number of possible nesting sites. But the water itself is getting cleaner."
On the first day of the census Monday, the "swan uppers" — those who lift the swans out of the water — made their way upstream in wooden rowing skiffs between banks overhung by willows and chestnut trees.
When a group of cygnets was spotted, an oarsman shouted "All up!" and the boats surrounded the family of birds. Two adults and seven fluffy grey young were quickly hauled ashore, weighed, measured and tagged. Within minutes they were back in the water, flustered but unhurt.
Perrins pronounced the group healthy — a good sign. This winter's severe floods inundated this stretch of the Thames and left a fast-flowing river that washed away many newborn cygnets.
This year's census continues until Friday, moving from Sunbury, on the outskirts of London, to Abingdon, 80 miles (130 kilometres) upstream.
Perrins has been part of the swan census since the 1970s, when the population went into sudden decline. He and his colleagues found the birds had been swallowing lead fishing weights, which were poisoning them. Once the weights were banned, swan numbers rebounded.
"The population has about doubled since then nationally," Perrins said. There are about 1,000 swans along this stretch of the river, still down from the 1950s' level.
The river has changed over the centuries, from a tranquil waterway to a boat-crammed working thoroughfare to a haven for affluent pleasure-boaters.
Most of the swan uppers are full-time watermen who work on commuter vessels, party boats or tugs. For them, swan upping is both an enjoyable working holiday and a link to ancient traditions.
In one of those rituals, the convoy pauses below Windsor Castle so boatmen can lay down their oars, stand and raise a toast to "the queen — seigneur of the swans."
"We're all very close," said 33-year-old Ross Hunter, a rowing coach and former Thames boat captain. "I know all the guys who do it. A lot of them have known me since I was a child. One used to babysit me. It's kind of a family."
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