This is the 18033rd most frequent Polish word.
żubrów
"Żubrów" refers to "of the European bison" or "related to European bison."
The word 'żubrów' is in the genitive plural form, used here to indicate possession by the 'herd' that I saw.
W rezerwacie widziałem stado żubrów.
In the reserve, I saw a herd of bison.
The word 'żubrów' is used in the genitive plural to describe the quantity related to 'a few' (kilka).
Na polanie pasło się kilka żubrów.
A few bison were grazing in the clearing.
The word 'żubrów' again appears in the genitive plural to connect to the habits being studied by scientists.