Magpies are a bit like Real Estate Agents, it’s all about TERRITORY TERRITORY TERRITORY.
A magpie will most likely only get to breed if it can find vacant territory. The young if they don’t get kept as a spare will be expelled to a bachelor flock on marginal land and wait.
Now the set up at our house is that we are on a corner and the intersection is the centre of their patch and it extends about halfway down each road. It is quite easy to spot the border as it is usually aligned with something like a fence, drive or school crossing. You will often see a large group ( 2 families ) walking slowly pretending to eat. Eventually if one bird decides to cross the border then it’s punch up time. This may eventually develop into arial combat.
The video shows the early stages.