Why flapping flight in birds require a lot of energy?

Flapping flight in birds requires a lot of energy primarily due to the need for continuous muscle contractions to power the wings against gravity and air resistance. Each wingbeat involves overcoming drag and generating lift, which demands significant energy input, especially during takeoff and sustained flight. Additionally, the rapid movement of the wings necessitates a high metabolic rate to supply the necessary oxygen and fuel to the muscles. Overall, the biomechanics of flapping flight, combined with the need for agility and speed, contribute to its energy-intensive nature.