EVERY time the state-of-the-art security alarm detects intruders at the Queen’s Balmoral estate, the police immediately suspect Arabella, Antoine, Parsley, Sage, Rose, Mary and Thyme.

Seven Indian runner ducks, bought by the Prince of Wales as organic, free-range pest controllers for his Scottish holiday retreat, are triggering dozens of false alarms as they roam freely around the grounds of Birkhall, the house the Prince inherited from the late Queen Elizabeth the Queen Mother.

Security at the estate has been upgraded in a £1 million overhaul, including the installation of underground sensors buried under paths and lawns. The system is now almost too efficient.

An estate source said: “The ducks are setting off the new alarms day and night. The seven of them going across the lawn is enough to trigger the system; the noise of the webbed feet on the ground is easily picked up by the sensors.”

Indian runners are extremely active, rarely brood during the day and, unlike most ducks, which waddle, they run.

First imported 200 years ago from the former Dutch East Indies, now Indonesia, they were originally known as penguin ducks because of their unusually tall and erect shape, which has been compared to a skittle or a Hock bottle.

They are also prolific egg layers, and, according to the Indian Runner Duck Association, will outperform any chicken. The Prince, an organic gardener, bought the ducks for £250 each to deter garden pests, as he does not believe in the use of chemicals.

Commercial breeders raise the creatures on wheat and duck pellets, but when the ducks are left to feed themselves they will eat worms, slugs, insects, grass and duckweed. Their liking for greens means that they help to keep the lawn at Birkhall trim.

The Duke and Duchess of Rothesay, as the Royal couple are known when in Scotland, are on their summer holiday at Birkhall, on the edge of the estate. They are said to fuss over the ducks as if they were family pets. They have even had a small wooden house made in the shape of a castle to keep the ducks safe from foxes at night.

Security at the 55,000-acre Balmoral estate, already tightened after intrusions into the grounds of Buckingham Palace, was stepped up even further after the bombings in London last month. During the Royal Family’s summer break this year, detachments of the Black Watch and the Royal Scots will form the Royal Guard, the first time that two regiments have been used to perform the role.

Last year’s bill for protecting the Queen at Balmoral was £1.2 million, with Grampian Police spending £23,000 a week on royal security. It will almost certainly increase this year.

The cost could be greatly reduced if the Prince considered the security system that saved Rome from the Gauls in 390BC, and which still protects many of Scotland’s bonded whisky warehouses from intruders. Instead of quacking ducks, he could try cackling geese.

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