In the early part of the 20th century, miners in Great Britain and the United States took caged canaries into coalmines in order to provide warning of the presence of toxic gases including carbon monoxide and methane. Canaries would visibly show distress and sway on their perches in the presence low concentrations of carbon monoxide before toppling over. The concept of the "canary in the coal mine" giving warning of a human health hazard is based on several principles. First, canaries were found to be more sensitive than both humans and other animals such as mice to the toxic effects of carbon monoxide. Second, the birds were allowed to share the same air exposures as the humans. Third, the occurrence of carbon monoxide poisoning in a bird was quite recognizable to the miners, since sick birds would tend to fall off of their perches and appear visibly ill. An article appearing in a 1914 issue of the Journal of Industrial and Engineering Chemistry provides a simple description of the concept (Burrell G, Seibert F. Experiments with small animals and carbon monoxide. Jl Indust Eng Chem. 1914;6:241--244.): Birds and mice may be used to detect carbon monoxide, because they are much more sensitive to the poisonous action of the gas than are men. Experiments by the Bureau of Mines show that canaries should be used in preference to mice, sparrows, or pigeons, because canaries are more sensitive to the gas. Rabbits, chickens, guinea pigs, or dogs, although useful for exploration work in mines, should be used only when birds or mice are unobtainable, and then, cautiously, because of their greater resistance to carbon monoxide poisoning. . . . Breathing apparatus must be used where birds show signs of distress, and, for this reason, birds are of great value in enabling rescue parties to use breathing apparatus to best advantage. For more on the use of animals as sensitive indicators of environmental hazards, providing an early warning system for public health, read the 2011Public Health Report - Animal Sentinels for Environmental and Public Health (http://www.publichealthreports.org/issueopen.cfm?articleID=2645 ) by John S. Reif, DVM, MSc, at Colorado State University, Department of Environmental and Radiological Health Sciences. This is clipped from the 1926 film by the US Bureau of Mines titles, Oxygen Breathing Apparatus. The film shows the kinds of breathing apparatus used in mine rescues and explains their various parts. A mine rescue team explores a mine, testing for low oxygen content and carbon monoxide. The entire film is posted to my channel. This US Bureau of Mines film and many others are available at the US National Archive in College Park, Maryland.