Hovering is a flight technique used by many bird species to spot preys. It consists of staying aloft in the same place. This maneuver is energetically expensive since wingbeats are intense. That´s why the type and size of the wing influences substantially upon the time of permanence in the air (see Glittering-bellied Emerald).
To hover it is essential to generate an ascending force (lift), which compensates for gravity so as not to fall to the ground, and to reduce thrust. With combined movements of the wings and tail birds balance forces to keep stable in the air.
|Photos 3-2008 © Amelia Besana|
When the American Kestrel spots something of interest it must nullify thrust. It positions in a vertical position lowering and spreading the tail. It regulates the wings with a greater or lesser angle of attack, which allows the kestrel to generate more lift and provide no thrust.