The classic example of a bird taken into a coal mine to detect toxic gas is the "canary in the coal mine". The idea of placing a warm-blooded animal in a mine to detect carbon monoxide was first proposed by John Scott Haldane in 1895., and canaries were used as early as 1896. Well into the 20th century, coal miners brought canaries into coal mines as an early-warning signal for toxic gases, primarily carbon monoxide. The birds, being more sensitive, would become sick before the miners, who would then have a chance to escape or put on protective respirators. In some cases, the canaries were kept in cages with dedicated oxygen tanks so the birds could survive after their illness provided a warning.

Source: Wikipedia