birds cann't fly

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my grandfather has a pair of birds,which were beening kept in a big cage.and i love them so much,whenever i put my hand into the cage to touch them they would "bite" my hand. one day, i touched them again,and sliped away from the cage, I worried about that i can't get them back, to my sprise that they can't fly as far as other wild birds do. finally, i caught them back. what's your idea/