[tutts]

Miners used to bring canary birds with them when mining. If they hit poisonous gas, the canary would die before the miners, alerting them to the gas.

The government can issue secret warrants to companies that they are not allowed to disclose, requiring them to hand over customer data. A warrant canary is a periodical statement from a company that they have not received such a warrant. The idea is that if they do receive a warrant, they will stop publishing the warrant, and the court can't compel them to.