Before the world has a single footstep of land-dwelling life, the fifth day brings a first wave of motion. Targum Pseudo-Jonathan on (Genesis 1:20) asks the lakes of the waters to swarm: reptiles, living animals, and the fowl whose "nest is upon the earth" but whose "way" is "upon the air of the expanse of the heavens."
Notice the division. Birds nest below but fly above. They belong to both realms. The Targumist does not flatten this; he honors it. Creation is not tidy categories. Some creatures bridge the elements — earth, water, and sky — and the Torah uses them to teach that borders are places where life flourishes, not where it gets confused.