According to the CIA:
Throughout history, trained animals have been used in security roles to fulfill mission requirements, notably by the armed forces, whether for transport, communication, or threat detection. From carrier pigeons in World War I to today’s explosives-detecting dogs, government agencies have turned to animals to do the important jobs human couldn’t do. CIA is no exception, and it once worked on developing ways animals could help with intelligence collection. This collection of declassified documents highlights the diverse programs involving the feasibility of using marine and avian animal capabilities in support of intelligence operations. For a variety of technical and other reasons, none of the programs ever became operational. Yet these documents provide a window into the innovative thinking applied to the intelligence mission aimed at countering increasingly sophisticated foreign adversaries.