Migratory birds build body fat before long-distance flights because fat is their main concentrated fuel reserve.
Before migration, many birds enter a period called hyperphagia, during which they eat much more than usual. Their body mass can rise substantially as stored fat accumulates, especially before journeys across deserts, seas, or other areas with few feeding opportunities.
Fat provides more energy per unit of mass than carbohydrates or protein. Birds use it during flight and can replenish the reserve at stopover sites. Some species also adjust their muscles and internal organs to meet the demands of migration, but stored fat remains the crucial energy source.
A common misconception is that birds simply fly continuously at the same pace. Migration usually includes a sequence of flights and stopovers, and weather, food availability, age, and the need to reach breeding grounds all influence how much fat a bird stores.