Tweety, my baby african lovebird

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I have been caring for african lovebirds for over ten years now. My first pair was able to produce baby chicks, but my brother separated them too early so they kept dying. Last year we bought two new pairs and one pair successfully produced one chick. I asked my friend who owns a pet shop as to when I should separate the baby chick from the mother lovebird. He said to just leave them alone and the chicks will just go out of the nest box on their own. Today I glanced at the nest box and to my horror saw the mother hen hurting the baby chick! I removed the baby chick immediately and is now in a shoe box. I wonder what triggered this behavior since we never really bother them and they are placed in a quiet place in my garden. To my fellow mylotters who also breed birds, what usually triggers this behavior? The baby bird's wings are bloody and had I not separated her, the mother was already starting on her head.