In the natural environment a parrot's routine daily behaviors should keep its nails quite sharp. That sharpness is essential for its activities (climbing, grasping, etc.). At the same time, keeping nails sharp should as well keep their length in check. It's like whittling the end of a stake to become pointed... it shortens it. The main reason captive parrots need their claws blunted is so we are able to handle them without having our skin breached by the tips.