I've been reading up (based on an argument) about mermaids and sirens. I've read Wikipedia entries about both creatures, but am getting confused where some people are using the term mermaid and siren intechangably. I understand from an etomogy stand point, French Italian and Spanish all use Siren to describe both. However, historically, is there a misnomer in modern English having two terms, or are they truly seperate creatures? Starbucks, for instance, calls it's two tailed mermaid the Siren, pulling it's companies name and logo from literature.
Since it's mythology, strictly speaking there's no such thing as a "misnomer", but I suppose the age of this style of depicting Sirens will affect whether people want to think of it as a mistake or not. Sirens with fish tails go back to the mediaeval period: piscine Sirens seem to have become particularly popular in or perhaps a little before the 12th century, when they start appearing with either one or two tails in book illuminations or among the carvings on the exterior of Romanesque churches.
Prior to that, and all the way back to antiquity, they were avine, that is, they had bird features, not fish features. In early Greek art (prior to 500 BCE or so) they were generally monstrous figures -- examples 1, 2, 3 -- and sometimes appear in company of other mythical monsters (note the Sphinx to the left in example 2). They also weren't always female. Later they're often depicted with musical instruments and generally have more appealing features -- examples 1, 2, 3, 4. By the Roman era they're positively desirable. Their increasing humanness probably has something to do with the increasing prominence of Naples, which kept up a major cult to one of the Sirens, Parthenope, for many centuries -- Naples (Neapolis, "new city") was a re-foundation of an older town named after her, and the name even means "girl-faced".
But they make sense either way, whether with avine or piscine features: they dwelled on rocks in the sea (identified in antiquity with the Sirenuse or Seirenoussai rocks south of Sorrento, just outside the Bay of Naples). From a certain point of view avine features make more sense, since they're supposed to have committed suicide (after having failed to seduce Odysseus) by jumping off cliffs into the sea. They don't seem to have been confused with other winged female figures, like souls (normally depicted with wings), Harpies (harpuiai, personified "snatching" winds), Nike ("victory") and so on.
Why, exactly, there was a shift to piscine features around the 12th century is not something I can answer. The New Pauly suggests that it was because they became conflated with separate (i.e. not from Greek myth) fish-women, described from the 7th century onwards; and reports that "The Ovide moralisé talks of both types, bird-S[irens] and fish-S[irens], jointly zeroing in on the seafarer (5,3451–3483)." (The Ovide moralisé is a 14th century French adaptation of Ovid; linked page is in French.)
