Canary

According to, "every miner need[ed] a canary." On Skaro, she used Clara Oswald as her "canary" to navigate the sewers beneath the Dalek City. (TV: The Witch's Familiar)

Behind the scenes
As the Eleventh Doctor explained to Dermot O'Leary in Dermot and the Doctor, miners sometimes used to take canaries down into the mines to check that the air was breathable.