The term "warrant canary" is a reference to the canaries used to provide warnings in coal mines, which would become visibly sick from carbon monoxide poisoning, warning of the otherwise-invisible danger.
Just like canaries in a coal mine, the canaries on web pages “die” when they are exposed to something toxic—like a secret FISA court order. Warrant canaries rely upon the legal theory of compelled speech. Compelled speech happens when a person is forced by the government to make expressive statements they do not want to make.
Fortunately, the First Amendment protects against compelled speech in most circumstances. In fact, we’re not aware of any case where a court has upheld compelled false speech. Thus, a service provider could argue that, when its statement about the legal process received is no longer true, it cannot be compelled to reissue the now false statement, and can, instead, remain silent.
So far, no court has addressed this issue. But if you’re not paying attention to a specific canary, you may never know when it changes. Plenty of providers don’t have warrant canaries. Those that do may not make them obvious. And when warrant canaries do change, it’s not always immediately obvious what that change means.
That’s why The Calyx Institute has joined with a coalition of organizations, including the Berkman Center for Internet and Society, New York University’s Technology Law & Policy Clinic, and the Electronic Frontier Foundation to launch Canarywatch.org
More at https://canarywatch.org