If they are impeding his movement, IMO, they need to be trimmed. If you are unsure how to or how much to clip, take him to an experienced bird groomer or vet, but it's a good skill to learn how to do at home. I personally prefer to do nail trims at home, as it's less traumatic for Kiwi and I can clip as I see fit. Kiwis nails do not get 'overgrown' (curling around), just razor sharp. I clip off the points so he is not mauling my hand/arm with them and leave them otherwise usable for him to grip stuff. Many vets and groomers take them down to short if you ask me. Even a very agile, strong adult can have gripping issues if their nails are clipped to short. They should have a slight 'hook' to them, but not be starting to "spiral" or get caught in stuff.