We examined grit consumption in the facultative granivorous Rufous-collared Sparrow (Zonotrichia capensis). Grit consumption fluctuated seasonally and was significantly correlated with morphological changes in the digestive tract, and with seed size. The highest values of grit consumption and digestive tract mass were observed during winter. Laboratory experiments suggested that grit consumption was a voluntary behavior rather than the result of accidental ingestion, and favored digestibility. Grit consumption varied considerably when Z. capensis fed on different types of food, but remained constant when food availability varied. We suggest that grit consumption together with morphological changes in the digestive tract allow Z. capensis to increase energy acquisition in response to higher energy demands during wintertime.