I think the picture in question ( big bird w/little horse in mouth) is in
Dr. David Norman's
book , ' Prehistoric Life ' . On page194 is a John Sibbick painting of
Diatryma with
Hyracoththerium in it's beak. The June ' 97 Discovery picture is of Titanis
walleri chasing an antelope. Hope this helps !
Regards, Truett Garner