Carrier pigeons were used in ancient times for relaying messages. But interest in the use of animals has changed with the development of microelectronics and miniaturization that allowed small listening devices to be put on birds and even small mammals.
More recently, technology has been catching up with dog-like robots for defense use as well as hummingbird-size drones tested by the Pentagon. The Air Force also has released video of "bugbots" or "birdbots" that could be used for surveillance and military applications, including potential swarm attacks.
Iran has a long history of suspecting animals for spying, particularly accusing the West of trying to gather information about its nuclear activities.
Back in 2008, two "spy pigeons" were suspected of being used to gather intelligence about Iran's uranium enrichment plant in Natanz, reported Iran's reformist paper Etemad Melli. It said one of the birds was captured not far from the heavily bunkered underground facility and had metal rings, strings and other suspicious features attached.