My iguana used to do the same with a not-so-friendly love bird of ours. The bird used to bite everyone except its main owner, my sister. The major problem we had was the iguana knocking the bird cage on its side. The funny thing was, the bird and my iguana turned out to be the bestest of friends, and when the bird died 5 years ago, my iguana actually went into a state of depression and wouldn't eat on his own for two weeks. But long before the two of them became friends, my iguana's tail did get bit once, and surprisingly, the tiny love bird did do some serious damage. Since a cockatiel is much bigger than a love bird, I'd be really careful.
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~*Chibi*~
Sir Iggy ( 1.0.0 El Salvador Blue Iguana), Ryo-ohki, Orion, Bud, & Finnegan (3.1.0 crazy kitties), Rosebud (0.1.0 cockerspaniel), and Kayko (0.0.1 leopard gecko)