This is the 7272nd most frequent Russian word.
воробьев
"Воробьёв" is the genitive plural form of "воробей," meaning "sparrow" in Russian.
Here, 'воробьев' denotes the plural accusative form of the noun 'воробей', which means 'sparrow'. The sentence describes the existence of sparrows on the tree.
На дереве живет много воробьев.
There are many sparrows living on the tree.
In this sentence, 'воробьев' is used as the plural accusative form of the noun 'воробей' as a direct object of the verb 'видели' (saw).
Мы видели стаю воробьев в парке.
We saw a flock of sparrows in the park.
Here, 'воробьев' is used in the plural genitive form indicating possession — the nest belonging to the sparrows.