In the wild, budgerigars live in flocks of hundreds or even thousands of birds. Birds within the flock will fight to establish the social hierarchy (pecking order). This same instinct appears when too many birds are kept together. Hens are usually more territorial than cocks.
The preferred perching for budgerigars will be that which is higher up – challenges to sit up high can cause fights. However, budgerigars are rarely aggressive by nature and outbursts will end quickly.