Pliny the Elder [1st century CE] (Natural History, Book 10, 67): We have been told of strange kinds of other birds in the Hercynian Forest of Germany whose feathers shine like fires at night-time... - [Rackham translation]

Gaius Julius Solinus [3rd century CE] (De mirabilibus mundi / Polyhistor, 20.3): The Hercynian forest produces birds whose wings, although night has prevailed and thickened the shadows, flash forth and shine through the darkness. Whence, the men who live there for the most part design their nocturnal excursions so they can use the birds for directing their journeys. The people cast them down in front on the dark paths, and the birds show the way by the guidance of their glittering feathers. - [Arwen Apps translation, 2011]

Isidore of Seville [7th century CE] (Etymologies, Book 12, 7:31): The hercynia is a bird named from the Hercynian forest of Germany where they are born. Their feathers sparkle so much in the shade that, however dark the night is with thick shadows, these feathers, when placed on the ground, give off light that helps to mark the way, and the sign of the glittering feathers makes clear the direction of the path. - [Barney, Lewis, et. al. translation]

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Birds 5.75): Lucidi are birds, as he says [i.e. Liber rerum], that are named because they give light [lucem dantes]; their feathers shine through the darkness in the night. Hence it is that the wings of those birds, when they wish, show the way by the grace of their bright feathers, when they lie down among the darkness. This bird is found in the forest of Hercinium in Germany, as Pliny says - [Badke translation/paraphrase]