LONDON — Britain’s royal swan census is back a year after it was put on hold because of the coronavirus pandemic.

The annual count of the swan population began Tuesday at Eton Bridge over the River Thames in Windsor, west of London. The Swan Upping is conducted each year to tally swans, and in particular their cygnet offspring, claimed by the British monarch.

The tradition, dating back to the 12th century, originally was a way for the monarchy to ensure a supply of swan meat to serve at feasts and banquets. These days, the census is carried out in the name of conservation.

David Barber, Queen Elizabeth II’s official swan marker, said that while last year’s count was canceled due to the pandemic, the majestic birds have faced increased threats with more people spending time by riversides as a break from lockdowns.

“We’ve seen more problems with shootings of swans in the lockdown period than we’ve ever seen before,” Barber said. “And we’ve had pollution problems, which has been higher than ever before.”

Getting caught in fishing tackle or attacked by dogs are other hazards swans face.

During the Swan Upping, cygnets are plucked from the water, wrestled to shore, measured, checked for disease and marked with a number.

A local conservation group, Swan Support, takes care of swans along the banks of the Thames. Swan Upping provides an opportunity to rescue swans ensnared by fishing lines, the group’s treatment and rescue coordinator, Wendy Hermon, said.

The count is taking place over three days this year, curtailed from its usual five.