Ovid [1st century CE] (The Metamorphoses, Book 5, 571): But why have you, Sirens, skilled in song, daughters of Acheloüs, the feathers and claws of birds, while still bearing human faces? Is it because you were numbered among the companions, when Proserpine gathered the flowers of Spring? When you had searched in vain for her on land, you wanted, then, to cross the waves on beating wings, so that the waters would also know of your trouble. The gods were willing, and suddenly you saw your limbs covered with golden plumage. But, so that your song, born, sweetly, in our ears, and your rich vocal gift, might not be lost with your tongues, each virgin face and human voice remained. - [Kline translation]

Pliny the Elder [1st century CE] (Natural History, Book 10, 70): Nor should the sirens obtain credit, although Dinon the father of the celebrated authority Chtarchus declares that they exist in India and that they charm people with their song and then when they are sunk in a heavy sleep tear them in pieces. Anybody who would believe that sort of thing would also assuredly not deny that snakes by licking the ears of the augur gave him the power to understand the language of birds. - [Rackham translation]

Isidore of Seville [7th century CE] (Etymologies, Book 11, 3:30-31): [Book 11, 3.30] People imagine three Sirens who were part maidens, part birds, having wings and talons; one of them would make music with her voice, the second with a flute, and the third with a lyre. They would draw sailors, enticed by the song, into shipwreck. [Book 11, 3.31] In truth, however, they were harlots, who, because they would seduce passers-by into destitution, were imagined as bringing shipwreck upon them. They were said to have had wings and talons because sexual desire both flies and wounds. They are said to have lived among the waves because the waves gave birth to Venus. - [Barney, Lewis, et. al. translation]

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Marine monsters 6.45): The sirens are loud animals, as Physiologus says, that from the head to the navel have the shape of a woman of great size, with a hideous face, and very long and scaly hair on the head. And they appear with their young, which they carry in their arms, for the mothers suckle the young, which they have at their breasts. And the sailors, when they see the sirens, are very much afraid, and then they throw an empty bottle to her, so that while she plays with the bottle, the ship escapes. This was testified by those who professed to have seen them. As for the rest, as Andelmus writes, they have a part of their body like that of the eagle, and claws on their feet capable of sculling; but at the end of their body they have the scaly tails of fishes, with which they swim like rowers in sea. They have a certain musical and sweet melody in their voice, with which the sailors, delighted and attracted, are sent into sleep, and the sleepers are torn apart by the claws of the sirens. These beasts are said to dwell on islands and sometimes in the waves. But some of the sailors, using a wise plan, block their ears strongly and thus pass unscathed, otherwise they would be disturbed by the deadly song of the sirens. These siren are not really beautiful, but as certain harlots, who led passers-by to poverty. They had wings and claws, because love flies and wounds. They dwell in rivers, that is, in the flow of luxury. Most people, however, and almost all the philosophers and even some of the expositors of the saints, disagree with Isidore, saying that we believe that the sirens are in truth monsters of the sea. Hence in truth we believe that they are monsters of the sea, but they are irrational: but the song is to be believed not articulated, to be distinguished by syllables and words, but certainly not distinguished by articulations like the song of birds. - [Badke translation/paraphrase]

Albertus Magnus [ca. 1200-1280 CE] (De animalibus, Book 24, 119): Sirens, popularized in poetic fable, are marine monsters whose upper body has the figure of a woman with long pendulous breasts with which it suckles its young; the face is horrible and it has a mane of long free-flowing hair; below they have eagle’s claws, and above are aquiline wings, and behind a scaly tail used as a rudder to guide their swimming. Upon making an appearance, they hold out their young in full view, emit some sweet, alluring sounds by which they lull their hearers to sleep, and then tear the sleepers to pieces. However, wise sailors plug their ears as they pass, all the while throwing overboard empty bottles to divert the sirens’ attention, until their ships are out of danger. - [Scanlan]

Bartholomaeus Anglicus [13th century CE] (Liber de proprietatibus rerum, Book18.96): The Mermaiden is called Sirena, & hec Siren, Sirenis, is a sea beast wonderfully shapen, & draweth shipmen to peril by swéetnes of song, & so this noune Sirena commeth of siren, yt is to understand, draught or drawing, as Hugution saith. The Glose saith super Esa. 13. G. that Sirene be Serpentes with creasts. And some men say, that they are fishes of the sea in likenesse of women. Papias sayth, that Sirene be great Dragons flieng with creasts as some men suppose. And Isidore saith li. 20. where hée treateth of wonders, in this wise, some men feine yt ther are three Sirenes semedeale maidens, and some deale Fowles with claws and wings, and one of them singeth with voyce, and another with a Pipe, and the third with an Harpe; and they please to shipmen with likenes of song, that they drawe them to perill and to shipwracke, but the truth is, that they were strong whoores, ye drewe men that passed by them, to povertie & mischiefe. And it is feined, that they brought them to shipwracke. And Isidore lib. 12. saith, that in Arabia be serpents with wings, that be called Sirene & run more swiftly then horses, and doe flye, and also it is sayd, that they flye with wings, and theyr venimme is so strong, that death is self sooner then ach or sore. And Phisiologus speaketh of Sirena, and sayth, it is a beast of the sea wonderfully shapen as as maid from the navell upward, and a fish from the navell downeward, and this wonderfull beast is gladde and merie in tempest, and sadde and heavie in fayre weather. With swéetnesse of song this beast maketh shipmen to sléepe, and when shée séeth that they be a sléepe, she goeth into the ship, and ravisheth which she may take with her, and bringeth him into a drye place, and maketh him first lye by her, and doe the déede of lechery, & if he will not or may not, then she slaieth him and eateth his flesh. Of such wonderfull beasts it is written in the great Alexander's story. - [Batman]