Why does a parakeet pull out their stomach feathers?

If you're bird is pulling out any feathers at all, make sure it is not stressed. Sometimes the bird is stressed. It could be from a harsh light, it being to cold/hot, or from being alone and neglected. Also, after mating there is a good chance any feathers will be pulled. A lot of times the bird is just preening itself and happens to pluck out a feather. It doesn't hurt them at all. It may even just seem like they are pulling out feathers, when they aren't. One just gets hit and falls out.