In the old days, a canary went down to work with coal miners. The practice was so ubiquitous that it's become a cliche. But why, specifically, is a canary such a good indicator of imminent suffocation?
The canary in the coal mine is always the first to go, whether in reality or in metaphor. It's a signal that something terrible is happening, whether it's financial disaster, medical breakdown, or political scandal. It's also a historical fact - miners did take canaries to work with them. If the canary died, they knew that they were soon to follow if they didn't run.
The idea of canaries was first proposed by John Haldane, a man who knew quite a lot about dabbling with deadly gas. He'd lock himself up with gas in a sealed room, taking copious notes on what it felt like to be slowly poisoned. Not that he didn't do good work. Carbon monoxide, a by-product of combustion, killed people in poorly ventilated spaces. Haldane noted the fact that the carbon monoxide combined with hemoglobin in the blood, and the resulting combination stained the tissues of the poisoned bright red. When coal miners mysteriously started dying, their faces flushed and red, Haldane put two and two together and realized carbon monoxide was the culprit. He suggested a canary be used as a carbon monoxide detector, and people did use them well into the 20th century.
Canaries, and birds in general, are suited to this not just because they're small and portable, but because their anatomy makes them vulnerable to airborne poisons. Birds are continuously "inhaling." This is what helps them fly, which is already a tremendously taxing aerobic activity, at heights that would cause a human altitude sickness. Human lungs house many little alveoli - sacs with thin outer layers that allow oxygen to pass into the bloodstream while letting carbon dioxide out of the blood stream and back into the lungs.
For birds, the oxygen goes in and the carbon dioxide out, when it travels through a structure that resembles a ribcage-like series of tubes. When a bird draws breath, it passes air through those tubes, absorbing the oxygen into its bloodstream while the remaining de-oxygenated air goes into two sacs in its body. It also takes in air that rushes directly to a second set of sacs. When the bird exhales, the "spent" air rushes out, along with the carbon dioxide. That second set of sacs, full of unused air, also empty. Their oxygen-rich air rushes through the tubes on its way out, letting the blood absorb yet more oxygen. Birds are getting fresh air when they inhale and when they exhale - a double dose for our single one.
This makes birds great at taking in oxygen, but extraordinarily sensitive to poisons in the air. A canary is taking in the poison twice with every breath. Even if it were as big as a human (and what a sight that would be), it would die faster from poisoning.
Top Image: flagstaffotos.com.au