Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Birds 5.20, 5.94): [Thomas describes the bittern under two names, butorius and onocrotalus.] [Birds 5.20] Butorius, as he says [i.e. Liber rerum], is a bird so called from the sound of its voice. It has long legs, an elongated neck, and a long and sharp beak like a heron. Only in color is it far from it, for it is colored very similar to earth. It lives around ponds, and is fond of eating fish. On the shore, under the heat of the sun, in a place where fish are known to frequent, it stands so still that, if you did not watch carefully, you would either believe it to be a dead bird or a motionless object. Hence it is that of unwary fish are less suspicious. But even though it has a long neck, it makes it short by recurving it, so that when the fish approach it more carelessly as if it had not thought of anything and was sitting at a safe distance, it draws them to him by the extension of its neck. When this bird knows that it is in a net, it pretends to be quiet. When an approaching fowler thinks he can catch it with his hand, it suddenly stabs with the arrow of its beak, with which it wields powerfully, and stabs him. As the Experimentator says, it has a long and very sharp beak, which it always raises to defend itself when sleeping and awake. When the hawk thinks it has caught the bird, it rushes to its death in its beak. It eats poisoned frogs. This bird alone in the springtime sings with a horrible voice. Nor is this said to be possible unless it immerses its head in water, so that the voice which it has conceived within itself, subdued by the element, may sound like a kind of thunder. When its flesh is put into the fire, it gives off a wonderful smell. Hence it happens that this bird often gives food to the rich man. Its skin is said to be useful in medicine. [Birds 5.94] The onocrotalus is a long-billed bird living in parts of the East. When it wants to enlarge its voice, it puts its head in the water and thus emits a cry like a roar. As the Experimentator says, it is not without reason that this bird has certain follicles in its throat, in which it first deposits its food and after an hour sends it into its second stomach. For it has two stomachs, one of which is in the throat, and in the other it cooks and digests food; so not in others. This bird has a long beak. But there are two kinds of them: one aquatic, the other solitary. This alone among the other birds lacks a spleen, as Aristotle says. Isidore says that this bird stores a lot of food in its stomach at the same time. ... And in this he means the avaricious, about whom Job says: He will vomit up the riches which he has devoured, and God will draw them out of his belly. But later - when the onocrotalus is hungry - it pulls out the food and eats it again. ... This impurity is forbidden by law. Scientists say that, by the example of this bird, Hippocrates learned the remedy of clyster. For as Ypocras was walking on the shore of the sea, he saw an onocrotalus bird constipated by the abundance of food. And when it was not able to run hither and thither, drawing salt water from the sea with its beak, it threw it into the back of his throat, and was soon relieved by a natural remedy with effect. Therefore this bird, as Pliny says, is a predator and lives on prey; in spring it cleanses the bowels, and this in a ruminating manner. - [Badke translation/paraphrase]

Bartholomaeus Anglicus [13th century CE] (Liber de proprietatibus rerum, Book12.28): The Miredromble is called Onacrocalus, and is a bird that maketh noyse in the Winter, and hath small chins in his jawes, in which hée taketh first meate, and then sendeth it to the second wombe: For he hath two wombes, in that one onelye hée taketh meate, and in that other onely he séetheth and defieth. But the first is taken in stéed of the crop of the throat, as Isidore saith. In Gréeke Onacrocalus is called a Birde with a long bill: and there be of two manner kindes: One is a water foule, & that other a foule of desart, and he that dwelleth in Water, is a bird of great gluttonye, and putteth the bill downe into the water, and maketh a greate noise, and is enimie namely to Eeles, & the pray that hée taketh, he swalloweth sodeinly, & sendeth it into his wombe. And then he cheweth and moveth his jawes, as he held meate in his mouth. This Bird resteth on the cliffe, and turneth upward his bellye to save himself against the réese of the Goshauke, that he maye in that manner the more safelye rest and sléepe, as Isidore sayth. - [Batman]