A. Unlike humans, bird bodies don't store fat. Fat, like their muscles, bones, and internal organs, have weight, and birds need to keep an ideal body weight in order to fly.
So on a day-in-and-day-out basis, birds eat on an energy-in-energy-out basis. They essentially consume food as needed. Throughout a frigid winter day, they eat from sun-up to sun-down just to have the energy to stay warm that night. All night long, they burn the calories (energy) they consumed that day. And the next morning, they start all over again.