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Secrets of Cold War spy-pigeons have been revealed after the CIA declassified sensitive files from the time.

The files from the 1960s and 70s reveal how pigeons were trained for clandestine missions photographing sensitive sites inside the Soviet Union.

The release also reveals how ravens were used to drop bugging devices on window sills and dolphins were trained for underwater missions.

The CIA believed animals could fulfil “unique” tasks for the agency’s clandestine operations behind the Iron Curtain at the height of tensions between Russia and the US.

(Image: SPY: The Secret World of Espionage)

The secret files are stored at the CIA’s headquarters in Langley, Virginia, is a museum, which is not open the general public.

The 1970s’ operation, codenamed Tacana, explored the use of pigeons with tiny cameras strapped to their bodies to automatically take photos, the newly released files show.

It took advantage of the fact that the humble pigeon can be dropped somewhere they have never been before andhave the amazing ability to find their way hundreds of miles back home.

The use of pigeons for communications dates back thousands of years but it was in World War One that they began to be used for intelligence gathering.

In World War Two a little known branch of British intelligence - MI14(d) - ran a Secret Pigeon Service which dropped birds in a container with a parachute over Occupied Europe.

A questionnaire was attached and more than 1,000 pigeons returned with messages including details of V1 rocket launch sites and German radar stations.

(Image: BBC)

One message from a resistance group produced a 12-page intelligence report sent directly to Churchill.

After the war, a special “Pigeon Sub-Committee” of Britain’s Joint Intelligence Committee looked at options for the Cold War but didn’t pursue the use of pigeons.

However the CIA continued exploiting pigeon power and also trained a raven to deliver and retrieve small objects of up to 40g from window sills of inaccessible buildings.

A flashing red laser beam was used to mark the target and a special lamp would draw the bird back.

The CIA secretly delivered a listening device by bird to a window sill although no audio was picked up from the intended target, the BBC reports.

The CIA also looked at whether migratory birds could be used to place sensors to detect whether the Soviet Union had tested chemical weapons.

Another operation called Acoustic Kitty involved placing listening devices inside a cat.

In the 1960s, the files show the CIA also looked at using dolphins for “harbour penetration” but found problems in handing over control from a trainer who had worked with a dolphin to a field agent.

By 1967, the CIA was spending more than $600,000 (£480,000) on three top secret programmes - Oxygas for dolphins, Axiolite involving birds and Kechel with dogs and cats.