MI5 plots attack of the radio-controlled pigeons - Telegraph.co.uk

"Sun spots are, of course, minute radio active particles thought how they affect the pigeons' homing instinct nobody knows.

"This gives some colour to the suggestion that pigeons might be able to home on an electric beam, in other words that you might have radio-controlled pigeons."

At the time, however, security services were sceptical that it could be achieved. "There is apparently a slight suspicion that a man in Scotland who is particularly successful is actually doing this," Liddell writes. "Caiger does not believe it."

It was not until 2007, that scientists were finally able to direct a bird's flight.

Scientists at Shadong University in China implanted microchips which plotted their course by sending electronic impulse signals

Previously-released MI5 files have referred to plans to train pigeons to carry explosives to fly into enemy searchlights.

Liddell also met colleagues to discuss impregnating papers with radioactive substances to set off an alarm if they were removed from a building.

Liddell wrote: "It is quite possible to impregnate paper, metal clips or ink with radioactive substance and to install either under the floor boards or in a door post, or under the ground outside an apparatus which will register if anybody goes out of the building with a secret paper so impregnated."

But snags would include health risks to anyone if the papers were left in a drawer, he noted:

"It would at the outset produce extreme lassitude and later a loss of blood counts.

"No serious harm would result if the papers were removed and the symptoms detected. To counter these ill-effects it would be possible to introduce some self-destroying material."