We characterized the seasonal composition and quality of migrant and non-migrant Pronghorn (Antilocapra Americana) diets in Yellowstone National Park during 2006–2007. During winter (January–April), when migrants and non-migrants occupied the same winter range, the overall percent relative density for each forage class in Pronghorn diets (n = 51 composite fecal samples) was 67 6% (standard error) shrubs, 17 3% forbs, 13 3% grasses, and 3 1% other. However, spring and summer diets differed for migrants and non-migrants. Diets of migrants (n = 34) to higher-elevation ranges with higher precipitation and forage quality during May–August were dominated by 68 2% forbs, whereas summer diets of non-migrants (n = 21) remaining on the winter range were co-dominated by 48 2% forbs and 42 1% shrubs. Diet quality for migrant Pronghorn, as indexed by fecal nitrogen and DAPA, was also generally higher than for non-migrants during a period when the demands of late gestation and lactation were high. These results suggest that improved perinatal condition among fawns born to migrant females in Yellowstone National Park may be driven by higher-quality forage conditions in migrant areas, bolstering conclusions from previous studies that migration represents an adaptive strategy in this population given current conditions in the Park.