The Ercinee are largely nondescript birds, what is known about it is that its feathers glow at night, and some accounts describe it as having gold and silver plating on its wings.
These birds resided in the Hercynian Forest (ancient and dense forest that stretched eastward from the Rhine River across southern Germany) and with the glow of its feathers —the pale speckles on their feathers light up with golden bioluminescence— it lighted the dark forest nights.
Ercinee were known to twitter if someone knowingly told a lie in its presence. The Ercinne makes frequent appearances in the works of Pliny (Gaius Plinius Secundus, a Roman author, naturalist, and natural philosopher), who indicates that if one should ask an Ercinee politely, they will guide the individual to where they seek to go.
According to other authors, in medieval times, the Ercinee were much sought after by German and French traders because an extract from its feathers could be used in producing glowing-in-the-dark inks. However, the Ercinee were not easy to come by, they were shy creatures, of an elusive nature, so mysterious as to be completely unknown in England and Scotland all through to the Middle Ages.
In the folklore and tradition of Germanic societies, the Ercinee feature as good luck omens. It is believed that seeing one at the beginning of a journey is a sign of good luck. Conversely seeing one on the return leg of a journey is seen as an ill omen.
Following the luminescent trail of an Ercinee –which mark its comes and goes and the location of its home— is also said to be bad luck usually bringing death to the foolish person who attempts it. Most people attempting to follow an Ercinee's trail back to safety will only find death falling into a dark cavern or off of a cliff, for occasionally the light from their feathers will go out, leaving anyone who follows it lost in the darkness.
-Yapp, W.B., 1987. Medieval knowledge of birds as shown in bestiaries. Archives of Natural History, 14(2), pp.175-210.
-Nigg, J. (Ed.), 1999. The book of fabulous beasts: A treasury of writings from ancient times to the present. Oxford University Press, USA.