Sagebrush is not obvious food. Its leaves carry monoterpenes and other secondary compounds that deter most herbivores, and it is evergreen but tough. Greater sage-grouse eat it anyway, and in winter it makes up essentially their whole diet, cropped from plants that stand above the snowpack.
The bird is anatomically committed to this. Unlike most gallinaceous birds it has a thin-walled gizzard rather than a strong grinding one, so it cannot process hard seeds and grain well. Chicks are different: they need insects in their first weeks, and forb-rich brood habitat matters for their survival.
The consequence is a conservation problem with no substitute. Where sagebrush is removed for cropland, energy development or invasive cheatgrass and the fires it feeds, sage-grouse cannot switch to another food, and populations across the range have declined substantially over recent decades.