But on his toughest-yet spy school test, he disappeared -- done in by some of his own kind: ravens.

The bird was a central figure in a decade-long US Central Intelligence Agency program to train animals as agents, helping Washington fight the Cold War against the Soviet Union.

On Thursday, the CIA released dozens of files from its tests on cats, dogs, dolphins and on birds from pigeons to some of the smartest: ravens and crows.

It studied cats as possible loose-roaming listening devices -- "audio surveillance vehicles" -- and put electrical implants in dogs' brains to see if they could be remotely controlled.

Neither of those programs went very far. More effort was put into training dolphins as potential saboteurs and helping spy on the Soviet Union's development of a nuclear submarine fleet, perhaps the most potent challenge to US power in the mid-1960s.