If the air became contaminated, the birds would show signs of distress, even falling off their perch. This early warning was vital.
إِذَا شَبَّ حَرِيقٌ فِي ٱلْمَنْجَمِ ،‏ كَانَ ٱلْعُمَّالُ وَرِجَالُ ٱلْإِنْقَاذِ يَأْخُذُونَ طَيْرَيِ ٱلْكَنَارِيِّ مَعَهُمْ حِينَ يَنْزِلُونَ تَحْتَ ٱلْأَرْضِ .‏