*takes a look at the lore becaus the only "bird men" I know are harpies*
Huh, very interesting. http://en.uesp.net/wiki/Lore:Father_of_the_Niben , http://en.uesp.net/wiki/Lore:Bird_Men
My opinion is that Topal the Pilot is pretty obviously inspired by the Odyssey, and so I'll take it to have about the same degree of accuracy.
Now I'm imagining this part, "After Topal's departure, the Bird Men are never mentioned again in recorded history, and are believed to have met their extinction at the hands of the Khajiit long before the Ayleids migrated to Cyrodiil." looking like this:
I think real but probably extinct.
Maybe a shared ancestry with harpies.
Seeing as so many other animal/human hybrids exist on Nirn I dont see why birds would be far fetched.