Air Pressure?

Question

When the swollows fly low is it because of high or low air pressure?

Answer 1

When gliding:

If we take a slice through a bird's wing its shape [cross section] is like a teardrop, which is called an aerofoil. When an oncoming stream of air hits the leading edge of an aerofoil it splits into two air streams, one passing over the top of the aerofoil and the other underneath. The air streams below the aerofoil bunch together forming a higher pressure region whilst those above spread apart to form a lower pressure region. The difference in pressure above and below the aerofoil creates lift.

At least thats how an aircraft wing operates.  How the bird actually produces forward thrust by the manner in which it  "flaps" it's wings, I do not know.