jats:pIn bone, the spacing between δjats:sup13</jats:sup>C in collagen and bioapatite carbonate is greater in herbivores than carnivores, with implications for understanding animal dietary ecology from surviving hard tissues. Two explanations have been proposed: varying diet composition or differences in physiology between herbivores and carnivores. We measured the isotopic effects of carnivorous and herbivorous diets on a single species, to test the effect of diet composition alone. Protein δjats:sup13</jats:sup>C and δjats:sup15</jats:sup>N and carbonate δjats:sup13</jats:sup>C were measured on egg and bone from hens on different diets. Herbivorous hens had a +14.3‰ spacing between egg albumen and shell δjats:sup13</jats:sup>C, compared to +12.4‰ for omnivorous hens, and +11.5‰ for carnivorous hens. The bioapatite–collagen Δjats:sup13</jats:sup>C spacing was measured as +6.2‰ for herbivorous hens, and calculated as +4.3‰ for omnivorous hens, and +3.4‰ for carnivorous hens—similar to observed mammalian herbivore and carnivore bioapatite–collagen Δjats:sup13</jats:sup>C differences. We conclude that a shift in diet composition from herbivory to carnivory in a single species does alter the bioapatite–collagen carbon isotopic spacing. Our data strongly suggest that this results from differences in the Δjats:sup13</jats:sup>Cjats:subbioapatite–diet</jats:sub> spacing, and not that of Δjats:sup13</jats:sup>Cjats:subcollagen–diet</jats:sub>.</jats:p>