A world of difference between chickens having boundies and a person setting such boundries for chickens.
Chickens do accept boundries but to look at such boundreis as an electric fence beyonf which the chicken wont venture would be a mistake. Chickens are naturally territorial. They will, given enough room, set their own boundries. Chances of them staying within these boundries depends on many things like food availability, cover spots, other creatures inside the boundries, dust bathing spots, hens outside the boundries the males might court and of course their own population fluctuations.
If one was to start with a boundry encompassing a couple of acres they may well stay within that. However, if there is something that interests them outside this boundry they will investigate. If the chickens don't find any obvious risks to kife and limb they may well visit such a place and if favourable, they may include the new area into their territory. Plenty of people keep free ranging birds and they tend not to wander past a certain distance once they've become familiar with their surroundings.
A few stakes in the ground hoping that the chickens will not touch those lovely brassica plants on the other side isn't likely to work for very long.