Learn Russian: Как раз вёснами перелётные стаи возвращаются из-за моря. - Exactly in springs, migratory flocks return from across the sea.

This is the instrumental of temporal iteration. While весной means "in spring" (often referring to a single season or the season in general), the plural вёснами means "during the springs" or "every spring." It specifically emphasizes that this action repeats regularly across multiple spring seasons.

I learned that из-за means "because of." Why does it mean "from across" in из-за моря?

The literal, spatial meaning of из-за + genitive is "from behind" or "from beyond." из-за моря describes the physical movement of the flocks coming from beyond the sea. The meaning "because of" is actually a figurative extension of this original spatial meaning!

Can I use the adjective перелётные to describe any migrating animals or people?

No, перелётный is highly specific to birds (and occasionally flying insects). It is built from the prefix пере- (across/over) and the root for flying (летать). For migrating land animals or humans, you would use a different word, such as мигрирующий.

What role does the phrase Как раз play in this sentence?

Как раз is a very common set phrase meaning "exactly," "just," or "precisely." Here, it adds emphasis to the timing: it is precisely during the springs that the return happens, as opposed to any other time of year.