I think it varies from cockatiel to cockatiel. When Cookie was an only tiel she was fairly clingy, but then she met the budgies and she liked to play with them as well as me. My tiel cage is in my bedroom, so she was alone while i was at school, and i felt bad so we got Bailee. Bailee is VERY clingy. He has to be on a person the whole time he's out of the cage, or at the very least he has to be able to see people from where he's sitting. He calls and whistles from me from his cage when he knows i'm around.