Στρίξ, στρίγες: the appellation in ancient Greek of the common owl; it was seen as a sinister and portentous creature, as opposed to the glaux, γλαύξ, which was that species of owl considered sacred to Athena and held as a symbol of wisdom. In folklore from the Classical period and perhaps earlier, the strix was believed to drink human blood for its sustenance; the origin of this belief is unclear, but it lasted from the Classical period at least into the Middle Ages*; moreover, this half-imagined bird is ancestor to some of the most notorious horrors of Europe.
In Romania, the strigoi were once much feared; they are better known to us as vampires. There are at root two kinds of strigoi: the living and the dead. The dead, or strigoi morti, are those familiar revenants we know from hundreds of books and films, who must drink blood to maintain their undead existence, whereas the strigoi vii, the living, are witches who drink blood to sustain their youth and unholy powers; of this latter type, Érszebet Báthory makes a fine exemplar. Both of these kinds of strigoi are able to transform themselves into animal shapes, and it is likely that their being named after the Greek strix is no mere simile, but originates in a belief that the terrible and evil owl could take the shape of a man. The strigoi morti arise from the corpses of the unmarried dead, from the remains of those who in life tasted human flesh, from perished infants born of incest; the strigoi vii are those who were children born with a caul, a startling inversion of the Western European belief that such a child is particularly blessed.
In Italian, too, the word for witch is strega; they were thought to wear animal forms, to consort, as is generally seen to be decorous among witches, with the Devil, and to drink the blood of children. Sadly, such an immense volume of nonsense is nowadays spoken in regard to these witches especially, even as compared to the witch in general, that it is very hard to find an untainted source on the folklore concerning them; these evil deeds fall back ultimately upon the pseudologous Aradia, the so-called Gospel of Witches, invented by the cretinous Leland. Not least of his crimes is the suggestion that the strega should have ever existed in reality, an assertion as bereft of sense as an earnest faith in werewolves.
Even in D&D, that fountain of modern cryptozoölogy, there are creatures currently described as a kind of four-winged bat mosquitoes which suck blood, called stirges; this is originally a simple metathesis for striges, and the origin of the creatures should be obvious. (Indeed, the further back through the editions you go, the more the stirge resembles a bird.)
It is difficult to speak with any certainty on such matters, but if there is a conclusion to be drawn from these abundant nightmares and spectres, it is perhaps: that the intolerable curve of a beak, alchemized by the human imagination, may be reduced to a salt that spawns multitudes. In this, I think, we are alone — the mouse fears the beak far more, but it sees only a beak.