This is the 1288th most frequent Russian word.
птиц
"Птиц" is the genitive plural form of "птица," meaning "birds."
The word 'птиц' is in its genitive plural form, used here to indicate 'many birds' as part of an object of observation.
На закате я наблюдал множество птиц, летящих к лесу.
At sunset, I observed many birds flying towards the forest.
In this sentence, the word 'птиц' appears in its prepositional plural form, pertaining to birds nearby.
Он заботится о птицах, которые живут рядом с его домом.
He cares for the birds living near his house.
Here, 'птиц' is in its genitive plural form to express possession, indicating 'songs of birds'.