Because your lungs get inflated like a balloon they become compressed when submerged in water and the air within is compressed. Air will not itself flow from one region of low pressure into a region of higher pressure. The diaphragm in one's body reduces lung pressure to permit breathing, but limit is strained when nearly 1m below the water surface. It is exceeded at more than 1,m.
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The difference in the concentration of a substance across a space is called a concentration [ equilibrium / gradient ]. ... If there is a concentration gradient, substances will move from an area of high concentration to an area of [ equal / low ] concentration
If it's two different kinds of birds it's called competition.