Parakeets have five different types of feathers on their bodies, and they periodically shed those feathers much the same way humans shed hair -- gradually, only a few feathers falling out at a time. Those feathers are quickly replaced with new ones. The process is called molting; it's the natural way the bird keeps his feathers in good shape for not only flight but also protection from the elements. This is one reason why parakeets preen so often, to keep their feathers working well.
Newly Hatched Parakeets
Baby parakeets come into the world completely naked; they have transparent skin that shows the developing pin feathers beneath. These form during the baby birds' first week. After the first week, feathers begin to emerge. Within four to five weeks, baby parakeets should be entirely feathered. The sheaths from the pin feathers will eventually crumble into dust on the birds' skin, which aids in keeping them clean during preening. The parent birds preen the young.
Parakeets molt at least once a year, but some molt every six months. Baby parakeets have their first molt around 12 weeks of age. Molting is symmetrical, meaning that the bird loses the same feather on both sides at the same time. Sometimes the molt takes weeks, sometimes a couple months. During that time the old feathers fall out, but not all at once, so the molt doesn't make the bird featherless or flightless.
Sometimes birds molt out of season. This is usually caused by stress factors, such as an abrupt change in temperature. Mating and breeding also cause unscheduled molts to occur, as can illness and parasites such as mites or lice, which irritate the birds' skin and cause the feathers to fall out in odd patterns. In these cases, the new feathers don't grow back in until the stressing factors have been removed from the birds' lives.
Sometimes birds who don't quite get along together will pick feathers from each other. This causes a great deal of stress on the parakeets and makes them bald if the picking doesn't stop. Feathers grow back in, in about four or five weeks, if the picking has stopped. Usually all it takes to stop the picking is to put one of the birds in a separate cage and allow the parakeets to heal naturally on their own.