Why is takeoff the most strenuous part of a bird's flight?

Takeoff is the most strenuous part of a bird's flight because it requires a significant amount of energy to achieve the necessary lift. Birds must overcome their own weight and generate enough thrust to ascend into the air, which often involves rapid wingbeats and increased muscle exertion. Additionally, during takeoff, birds must coordinate their movements precisely to ensure stability and control, making it a physically demanding phase of flight.