In bird names, pied means the bird is black and white, in patches, like a magpie. A pied wagtail, a pied flycatcher, a pied kingfisher — the word is telling you the colours, not a different species. It comes from an old word pie, which meant magpie; people looked at that bird’s mix of black and white and used the same idea for other birds. Though actually, older English also used pied for clothes of mixed colours, not only black and white: the Pied Piper of Hamelin is “pied” because his coat was patched with many colours, not because he was a bird. In modern English you mostly meet the word in those bird names, and in that one famous story.