As you know every bird that can fly has wings. The shape a birds wings creates is called an airfoil. As birds fly, air goes above and below the airfoil. Air has to move father on the upper surface of the airfoil than the lower part of the airfoil, for two air flows to make it to the end of the bird's wing at the same time. This makes the bird's wings lifting off the ground possible. In order to make lift, speed is the most important factor. When a a bird doubles its speed, it doubles its speed. That also means if a bird triples its speed it can have 9 times the lift. In order for a bird to obtain speed (have more lift) the bird needs to flap it wings. For a bird to slow its speed down (have less lift) the bird needs to flap its wings less.

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Many birds like vultures can soar. Many birds that soar use upward air movements that are called thermals, these birds rest thier wings on ar that is moving upward. These vultures use these thermals as a valuble resource for soaring. Vultures and other birds that soar are constantly rising in top of a current of air, and at the same time gliding downwards. Here is a video with a glimpse of many vultures soaring.

Another kind of flight many birds use is, "taking off," flight and "landing flight," these two forms of flight are usually the most dangerous or difficult for birds. Taking off is difficult for birds because it is hard for birds to attain lift, because their speed is so slow. (Less speed, less lift--more speed, more lift) It is also difficult for birds to land because the speed of the bird is getting slower. Another factor that determines lift besides speed is the size of the birds wing. The angle that the wing hits the air determines lift also. Here is a video of birds successfully taking off from the ground using proper speed, and wing angle in order to maintain lift.

For more info on the science of bird flight, here are some other sources:
1. http://www.livescience.com/animals/041209_birds_fly.html
2. http://www.ornithopter.org/birdflight/scifair.shtml