A nice summary of the behaviour of wild budgerigars, from the Encyclopedia of Life:
"Budgerigars aggregate into large flocks and are strongly social. Their grouping allows for greater success in feeding and also helps in protection from predators. There does not seem to be any hierarchy in groups based upon the relatively few battles among individuals, but females are generally more aggressive than males.
Their activity, like most birds, begins just before sunrise with preening, singing, and movement within trees. After sunrise, the birds fly to the foraging area and feed throughout the day. They do not forage during midday or in extremely hot weather, instead they take shelter under shade and remain motionless. At the end of the day, they congregate by calling loudly and flying at high speeds around the trees. They then return to their roosting site just after sunset and remain at rest until the next morning (Kavanau, 1987)."
Image (c) Karenstock on Flickr