Sources : Wagtail

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Birds 5.67): Isopigis, or cropa, is a bird, as the Liber Kyrannidarum says, of a gray or sometimes black or variegated color. It always shakes its tail while sitting by the brooks, hence its name of the quivering tail. And it signifies those who, being subject to a vague and unstable affection, yet in no way shudder at the memory of death, and this with the human condition contemplates the unstable flow. And in the book of Kings it is said: We all die and slip away like water. - [Badke translation/paraphrase]