"fowl" means bird, and a raven is a type of bird. so, simply put, the author is just referring to the raven with a different term. "ungainly" means awkward or clumsy, so I assume the raven he's speaking of is moving about in a clumsy sort of way?
I'm not sure if that's the answer you wanted, if not let me know and I'll try to help as best I can.
