As the wing is pulled upwards again, the placement of the feathers allows air to pass between them, so that drag is reduced. The innermost feathers of the wing are the secondary and tertial feathers. These feathers form an airfoil in cross-section, a curved shape that allows air to move faster over the longer upper surface, providing an upward lifting force. Gliding is the simplest method of flight, relying on the airfoil shape of the wing to generate lift. Birds like hawks and seabirds glide extensively, and search for upwards moving air currents or 'thermals' in the same way as a gliding airplane.
Most birds use a combination of gliding and beating of the wings to keep airborne.
Because the wings are controlled by muscles and are flexible, bird flight is one of life's marvels - birds intuitively know how to dive, swoop, turn and brake in a way that no airplane can. Some birds, like the hummingbirds, can even hover or fly backwards, with their wings beating at up to 50 times a second to keep them in the air. When you think of a bird's senses, you probably think of one of two things. First, you may think that birds have little intelligence, possibly because one of the stereotypes of a bird is an animal that runs after brightly shining objects, or which imprints itself on the first thing it sees after emerging from the egg. Second, you may think of a bird like a hawk or owl, and marvel at the ability that these predators have of stalking their prey, using acute hearing and eyesight.