British secret services debated the idea of "radio-controlled pigeons", newly-released records show.
The post-war diaries of Guy Liddell, then deputy director general of MI5, show that in the aftermath of the Second World War he discussed the idea with Captain James Caiger, who ran the Army's pigeon loft after the war.
In an entry for October 3, 1946, Mr Liddell described how Capt Caiger came to see him. "He is our pigeon expert. He is, in fact, the nearest thing to a pigeon that I have ever seen. He talks, thinks and dreams about them," he wrote.
"He has had pigeons since he was a boy and his father had pigeons before him.
"I asked him about the homing instinct. He said that the matter is quite unsolved.
"There is however, one curious fact, namely that in a sun spot year all pigeons go hay-wire.
"Sun spots are, of course, minute radioactive particles though 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."
Previously-released MI5 files have referred to plans to train pigeons to carry explosives to fly into enemy searchlights.
Mr Liddell's diaries, released to the National Archives in Kew, west London, refer to other "spycraft" ideas.
In February 1949 he met colleagues to discuss impregnating papers with radioactive substances to set off an alarm if they were removed from a building.
Mr Liddell wrote that he was told: "It is quite possible to impregnate paper, metal clips or ink with radioactive substance and to install either under the floorboards 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."