My cockatiel is about 9 years old. Lately (past 2 or 3 days) he's been unusually happy and excessively friendly. This is very unusual behaviour for him and I'm wondering - could he be sick? He usually doesn't enjoy human contact and is very quiet and keeps to himself. Now he's very happy, making lots of noise and moving all over the place. This is very plesant, but it's so out of character I'm worried about him. Any thoughts?