Toe Biting

Schnitzel has just today developed an obsession with biting his toenails. I'm sure it's the actual nail he is biting because I can hear the clicking of the beak against it. He doesn't appear to be in any pain and nothing different or traumatic has happened to him. It looks as though he is just picking at them but I wasn't sure if it was something to worry about.

I read on one of the other forum topics that budgies who are 'highly strung', stressed or bored can do this. He is taking a very long time to tame, he is still scared of us- could this have something to do with it? He has both room in his cage and of out of the cage time and he has toys that I change and rotate regularly(even though he has never touched one- he really is a different little bird!)

What could be causing this and what can I do?