The remains of a swan have been found barbecued near Queen Elizabeth II's castle in Windsor, police said.
By law, all unmarked mute swans in England and Wales belong to the queen.
The swan had been killed and burned over the weekend, Thames Valley Police said. Its remains were found Sunday by a local council officer, who contacted police.
The incident is being investigated as a theft, a police statement said. Anyone with information is urged to get in touch.
Wendy Hermon, of the charity Swan Lifeline, told CNN of her disgust and puzzlement at the crime.
"It's an act of vandalism which shouldn't happen," she said. "The most sickening thing about all this is the swans will go to people because they think they are going to feed them. ... I always say they are semidomestic because they trust us, so it's going into a trap."
The majestic white birds are a fairly common sight on the River Thames, which flows past Windsor and on to London.
According to the official website of the British monarchy, all mute swans have belonged to the Crown since the 12th century, when they were regarded as a delicacy and eaten at banquets.
"Today, the Crown retains the right to ownership of all unmarked mute swans in open water, but the queen only exercises her ownership on certain stretches of the Thames and its surrounding tributaries," it says.
Each year, a census is carried out of the swans on stretches of the Thames in the counties of Middlesex, Surrey, Buckinghamshire, Berkshire and Oxfordshire, it says. This practice is known as swan upping.