Birds have a unique set of physical characteristics that work together with its wing to allow them to take flight.
Some of those distinctive features include:
Hollow bones which are light and durable.
Feathers which are light and designed to catch air.
Incredible lung capacity which is very efficient and processing oxygen which enables it to go long distances without being tired.
Consumes an abundance of high-energy food.
If you have ever put your hand outside the window you more than likely have noticed that your hand kept moving up and down depending on the way you tilt it.
If you turn your hands downwards, it tends to go down against the wind.
Alternatively, if you tilt your hand upwards so as to cup the wind, your hand moves up as a result of the air pushing against it.
When a bird is in flight, air flows around it quite easily similarly to when you put your hand outside the window.
However, the difference is that as air flows over the wing, it flows faster at the top, than it does at the bottom due to the fact that the wings of birds are curved at the top.
What this means is that they get more airflow on the bottom side of their winds.
When this occurs, it gives them a push and since their wings are flat, it gives them a lift.
So in essence, as a bird slices through the air, the physical characteristics of its light body and wings, causes it to get pushed upwards from below the bottom part of their wings which keeps them in flight.