My atiel is 17 wks old and gets angry sometimes?

Hi all. i just got rico my atiel when he was 6 weeks old. he is now 17 weeks old and i love him but sometimes he gets angry. He will be on my shoulder and i go to scratch his head which he usually loves and he makes a shrieking noise and opens his beak and hits my finger but does not bite. is his normal or will he grow out of it. I also think he is starting to malt, could that bey why??thanks