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Birds have feathers which serve different purposes--flight, insulation, waterproofing and even for sexual attraction. Flight feathers are long, stiff and found on a bird's wing or tail. They enable a bird to catch the breeze and provide both lift as well as thrust or forward motion. Lift is provided when due to the shape of the bird's wing, air passing over the top has to travel a longer distance and has to speed up as it goes over the top, relative to below the wing. This creates a difference in pressure and provides lift. When a bird flap's its wings using its breast muscles, air moves over and under the wing--this is called thrust. The shape of the wing and its ability to move through the air help a bird to fly. Once in the air, a bird can conserve its energy and glide while looking for food. Hollow bones and a low body weight, and the fact that a bird has no urinary bladder, all keep the weight of birds low enough to achieve flight.
The birds behave like a glider. When they reach a certain altitude, they will ride on warm air currents which will provide the birds with lift, so the birds do not have to constantly flap their wings until they stop "riding" on the warm air currents.
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