This is the 12651st most frequent Polish word.
niebie
"Niebie" in Polish means "sky" or "heaven" in the locative case.
In this sentence, 'niebie' refers to the location where the birds are flying.
Ptaki latają wysoko na niebie.
The birds fly high in the sky.
Here, 'niebie' is used to describe the place where clouds are observed.
Widzę chmury na niebie.
I see clouds in the sky.
In this context, 'niebie' indicates the nighttime sky where the stars are visible.