This is the 16179th most frequent Polish word.
umierały
"Umierały" means "they were dying" in English.
The word "umierały" is used here as a past habitual action, describing the birds' regular occurrence of dying during the winter conditions.
Ptaki umierały z głodu w surową zimę.
The birds were dying of hunger during the harsh winter.
In this sentence, "umierały" is used metaphorically to describe the disappearance or loss of something concurrent with another event.
Niektóre opowieści umierały wraz z wiekiem dziadków.
Some stories died along with the grandparents' age.
Here, "umierały" literal and depicts an ongoing past process of wilting or dying due to lack of water.