What forms the overwhelming majority of the White-crowned Sparrow's caloric intake during the winter months?

Seeds and grains

During periods when the White-crowned Sparrow is not actively nesting, typically throughout the winter months or non-breeding periods, its diet is fundamentally structured around seeds and grains. This dietary choice is a deliberate energy management strategy tailored to the challenges of surviving cold weather and undertaking long migratory journeys. Seeds are rich sources of dense carbohydrates and fats, providing the sustained, slow-burn fuel source necessary for maintaining body temperature and enduring periods of scarcity. These seeds are primarily sourced from grasses and weeds, which remain available on the ground across their North American range during late fall and winter. This reliance on seed consumption ensures caloric density when the high metabolic costs associated with reproduction are absent.