You might think that this Whistling Kite is able to spend so much of it's life flying because it learnt how to fly... how to turn, climb and dive and glide much like when I was taught to fly aeroplanes. With an aeroplane it is throttle, stick and rudder, with a bird, flapping of it's wings and adjusting it's tail.
Bird's, however, instinctively need to apply a strict energy management policy during flight to ensure their survivial, whether it's a falcon using its height and speed to stoop on it's prey or an albatross patrolling over the ocean.
The Whistling Kite above has a perfectly balanced energy equilibrium; by using its height above the ground, the correct speed and aerodynamic streamlining to maximum effect it can fly searching for food with the minimum expenditure of energy. By using updrafts from thermals or from winds blowing across hilly terrain it can gain energy at no expenditure to itself. This "free energy" is especially important to birds such as eagles, kites and pelagic seabirds that spend much of their time airborne.
Unlike the photo above, these two photos of the same individual Kite demonstrate it depleting its energy supply rapidly. It was taking off from a temporary perch and are dramatic examples of an energy intensive activity where it's energy management is in a negative state.
The Kite is using maximum energy by powerful downward movement of it large wings to propel it into the sky. It could not continue like this for too long before it would become exhausted. Energy management per se strangely enough is not commonly taught in flight school for human pilots. Fighter pilots that survived more than the first few battles learnt very quickly how important it was... in a dogfight pilots needed to manage their potential energy; height and speed, throughout the entire engagement to avoid being shot out of the sky. Like birds, once a fighter pilot lost that advantage their chances of surviving were much reduced.