How does earth maintain a constant level of oxygen?

It doesn’t! the oxygen level of the planet has varied quite dramatically in the last 500 million years ago; as the climate cooled and the land plants died off, oxygen level fell to as low as 12 percent by the beginning of the triassic. Back then, the air at sea level would have felt thinner than at the top of the Alps today.

Burning fossil fuels has reduced oxygen levels very slightly  - about 0.057 percent over the last 30 years. Deforestation only has a small effect because when rainforest is cut down, other plants are usually grown in its place. But it’s marine plankton, rather than trees, that produces about 70 percent of atmospheric oxygen. Global warming will have a significant impact on plankton, which is a much more serious threat to oxygen levels.

DO  BIRDS SUFFER  FROM  TURBULENCE  WHEN  FLYING?

Birds normally fly close to the ground, around trees and buildings, or soar in the rising air of thermals. Both are high-turbulence environments, but birds have two adaptations that make them much better at coping with it than planes. Nerves at the base of their feathers allow them to detect the flow of air over their wings and monitor turbulence very accurately. Instead of waiting for an air gust to buffet the entire wing, they can anticipate each minor eddy and continuously make tiny adjustments. Large soaring birds, like eagles and vultures, can also quickly tuck their wings downward and coast through a small patch of turbulence. Each tuck last just a third of a sudden loading on their wings.