Phil Bigelow sent some useful looking refs re such birds - thanks a lot! - and also mentioned the Discover mag pic of the horse waving big bird, which I hadn’t seen - but -
Truett Garner said:
< 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. >
Yes - that’s it! Taa muchly - Fantastic pic. - good artist!