When methane built up, or carbon dioxide built up, or oxygen was diminished, the birds would swoon before people would -- so it acted as an early warning system: Hey, get out of the mine. Things aren't going so well.
Quando o metano ou  o dióxido de carbono se concentravam ou o oxigénio diminuía, os pássaros  desmaiavam antes das pessoas — era um sistema de aviso prévio: "Olhem, saiam da mina.