In a somewhat older post from HP Mickey Mortimer about the Mesozoic birds in Russia and Mongolia (based on the excellent book "The Age of Dinosaurs in Russia and Mongolia), he said something about a volant Hesperorn based on the internal large holes of the metatarsus. Better check the archives, it is in there. Not that the animal is known from a great deal of material...after all, we are here speaking about _Russian_ and _Mongolian_ bird fossils. ;)
Rutger Jansma