The question is really all in the heading: is it possible for a macroscopic organism to subsist entirely on rocks and still gain enough calories to survive? I know of Lithotrophs but all known lithotrophs are bacteria or archaea.
My idea at present is that existing lithotrophs were incorporated into the gut bacteria of animals that swallow stones to help digestion, and as they eat part (or all) of the stone they thrive and also aid the creature eating the stone. So I've figured out how and why they eat rock in the first place; could any macrofauna get enough calorie for life purely from eating rocks? If not, how many calories could they get? (So I can figure out what else they eat to make it up)
If it matters, the creatures in question are flightless birds around the size of a grey heron.