The force of gravity is greater than the force of lift that their wings can generate-animals have adaptations that allow them to live more easily in their environment. in the case of birds, there are two major adaptations that make them suited to flying. the first is the hollow bones that reduce the mass of the bird, and thus the gravitational pull downward. the second is the shape of the wings. bird wings follow a similar shape to those of an airplane, except that the bottom side of the wing is not filled in. because the air passing over the wing must travel over a longer curve than those below the wing, the air pressure above the wing is less than that below as faster moving air above the wing reduces air pressure. the higher pressure beneath the wings causes the bird to gain lift.it is believed that many species of flightless birds became flightless as their natural predators became extinct or competition for resources became less fierce. as the life pattern of the ostrich changed, its bones began to become heavier and the wings to become too small to lift the heavy mass of the bird. the modern ostrich can no longer fly because the lift produced is not sufficient.
