He was surprised to see fenced-in ducks on the factory premises. When he asked why, he received the following answer: “If the ducks keel over, there’s still enough time to evacuate people from the plant.”
The explanation rests on in the fact that heavy gases collect at the bottom and only rise slowly to the top. As animals were more sensitive and closer to the source of danger, they were used as a kind of sensor or early warning system.
We know something similar from the mining industry. From the late 19th century until the mid-20th century, canaries were used as an early warning system for carbon monoxide and other toxic gases. This practice began in the UK and was adopted internationally, including in Germany.