Are Harpies Sirens?

 According to the notice, the tomb of Kybernis in the British Museum, dating from about 480 BC, has long been known as the Harpy Tomb after the female-headed birds on the four corners.  The British Museum uses the terms "Harpy" and "Siren" interchangeably, as though they were the same thing.  But are they? 

 

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