The story starts a long, in fact very long, time ago, with the evolution of the first dinosaurian ancestors of birds during the Triassic. Life on earth was still recovering from the greatest mass extinction event ever, the end-Permian event, when perhaps 95% of all the species of living things on earth died out. The survivors at first had little competition, but as new species evolved the evolutionary arms race intensified.
One of the areas of competition was in physiology. Active animals with a high metabolic rate could be up and about, finding food and catching prey, while less energetic animals were unable to escape or compete. To run an active metabolism however, an animal needs to be able to absorb oxygen efficiently from the air, and two methods eventually dominated.
|Bird lungs - inhalation above, exhalation below|
The efficient avian lungs however have one drawback. They do not just absorb oxygen from the air, but inevitable accelerate the absorption of other gases. As a result, birds are highly susceptible to toxic gases. In many cases of the unexplained death of a pet bird, toxic fumes, often from an overheated non-stick pan, are the actual cause.
How birds came to be used as living gas detectors is down to one of the most famous scientists of the 19th Century, JS Haldane. A brilliant investigator, and dedicated Socialist, he devoted a great deal of time to the benefit of working men, especially miners. As part of his investigations of mining disasters and their causes, he analysed the various gases (mainly methane and carbon monoxide) which were so lethal , especially in coal mines. After experimenting on mice and birds, he suggested that each mine should have some canaries on hand in case of trouble. A canary could be taken down to the suspected trouble spot and if it showed signs of distress the miners would know to get out and improve the ventilation in a hurry. The scheme caught on, and even after mechanical devices began to take over, canaries were still kept on hand in case of emergencies.