As a former trainer reveals, the U.S. government deployed nonhuman operatives—ravens, pigeons, even cats—to spy on cold war adversariesThere would be a rustle of oily black feathers as a raven settled on the window ledge of a once-grand apartment building in some Eastern European capital. The bird would pace across the ledge a few times but quickly depart. In an apartment on the other side of the window, no one would shift his attention from the briefing papers or the chilled vodka set out on a table. Nor would anything seem amiss in the jagged piece of gray slate resting...