This is the 17786th most frequent Polish word.
zwiastunami
"Harbingers" or "heralds."
In this sentence, 'zwiastunami' is used to mean signs or indicators that something (autumn) is approaching, showing predictive characteristics.
Kolorowe liście są zwiastunami jesieni.
Colorful leaves are the harbingers of autumn.
Here, 'zwiastunami' denotes signs or early indications warning of forthcoming difficulties, emphasizing an ominous premonition.
Niepokojące wieści były zwiastunami nadchodzących problemów.
Worrisome news were harbingers of upcoming troubles.
In this case, 'zwiastunami' describes signs indicating the arrival of spring, reflecting positive and welcoming signals.