Biting may also be inadvertently taught when the new owner reaches his/her hand to pick up the baby parrot. The young parrot will usually reach toward the hand with its beak as a way to grasp it and climb on. If the owner pulls his hand back too quickly, the baby realizes that to get picked up, it better grab quicker next time. And soon grabbing can turn into biting.
Fear: 'Fight' or 'flight' are the common mechansisms used by animals if they are afraid or hurt. Birds in the wild will generally take to flight if they are startled or afraid, and biting would rarely be used. Birds in houses, with clipped wings, however, do not have that as an avenue. They may bite if startled or hurt.
Control or Dominance: As we all know, groups of birds tend to have a pecking order, and the birds we have as pets are often no different. A bird may use biting as a way to defend his territory, and thus his status. This 'territory' may be a cage, another bird, or the human being to whom he is bonded. Some birds soon learn that if they bite, they get what they want, be it the drama of seeing their owner dancing around and yelling in pain, or a trip back to the cage where they can eat. Birds may also learn to bite to avoid doing something, such as getting their nails trimmed or being put back in their cage when they want to stay out.
Breeding behavior: For many species, sexually mature adult birds in breeding mode can become aggressive and much more protective of their mate and more likely to defend their cage as their area. It is important to distinguish if biting behavior may be hormonally driven and more of a 'phase', or if the biting is a sign of dominance aggression.
Medical: If a bird does not feel well, they will often want to be left alone, just as we do. If biting starts in a bird who normally does not bite, or if you see other behavior changes such as eating or playing less, have your bird examined by a veterinarian.
Playing: During the course of play, a bird may occasionally bite. This can occur if the bird is overexcited, or the bird inadvertently grasps some part of human anatomy to keep its balance.
Learn to read the body language of birds. They will often show you how they feel. For instance, they may show signs they've had enough play or 'outside' time and need to go back to their cage. If you do not recognize this, the bird may start to bite to signal he's 'had enough'.