Aside from being the father of Man beasts or ..were - creatures of all shapes and sizes to reshape mortals in his image.

Did Hircine create the Hargravens or just use them to do his bidding?

They, apparently, traded their humanity for some serious magic skillz. Related quest exists, called Repentance. I don't recall doing it so I have no idea how much does it explain, if anything at all, but you might check it out.

Its a mutually beneficial arrangement more then anything else. The Hagraven's more then likely offer something up to Hircine, and then they get knowledge/transformation which allows for them to be...well, Hagravens.

The ritual involves a human sacrifice apparently, but that's about all we get to, given that most PC's aren't down with being offered up to create a deranged bird-lady. Oh, and the daughter isn't on friendly terms with her mother at that point either.