Sources : Nightjar

Pliny the Elder [1st century CE] (Natural History, Book 10, 56): Those called goat-suckers, which resemble a rather large blackbird, are night thieves - for they cannot see in the daytime. They enter the shepherds' stalls and fly to the goats' udders in order to suck their milk, which injures the udder and makes it perish, and the goats they have milked in this way gradually go blind. - [Rackham translation]

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Birds 5.5): Agochiles is a bird of the Orient, which is known to have a name from the fact. For Michael [Scotus], who translated Aristotle's book De animalibus, says that it is called agothilez in Arabic and Latin, suggesting the milk of goats. It is a large bird that lays two or three eggs. This bird asks the goats for milk, and supposes itself to be watered by their teats, and with its broad beak which was created for this purpose, it sucks milk from the teats like a goat. And this is very surprising in them, because when they have suckled the udders, the milk in them dries up and the udders of the goats dry up. Pliny also says, that the goat loses its sight from the sucking. And he calls these caprimulgas. - [Badke translation/paraphrase]