Inappropriate or understimulating toys can lead to inactivity, which can stimulate fear responses. A bird that doesn't play is more likely to exhibit increasingly introverted behaviors, including skittishness or fearfulness. Additionally, a parrot kept in a poorly designed or poorly located cage with insufficient light, inadequate diet or insufficient sleep might be inclined to revert to fear responses. By the time he is six months old, a healthy parrot is curious, active and exploratory. If a bird isn't curious and doesn't develop some sense of independence during the first year, he is at risk of developing behaviors that progress beyond caution to fearfulness.
For more information, please read the article The Fearful Parrot.