They'll establish their own territorial limits without our help. That's part of why they like roaming.
If you care about native bird and small mammal populations then don't introduce a cat into the environment. I used to live in an area with a lot of endangered native rodents and while I would've liked to have a cat around I didn't get one. It's not complicated.
Additionally, feral cats are awful for native bird and small mammal populations. Your cat will probably hunt and eat small mammals and birds if you let it roam freely.
https://en.m.wikipedia.org/wiki/Cat_predation_on_wildlife