I have a 1-year-old female cockatiel. She puts her rear against the bird cage with her tail in the air and goes into her own little cockatiel world, quietly squawking. And by "own little world,” I mean that you can actually touch my cockatiel without her being frightened.
Your female cockatiel is in her own little world. The posture you describe sounds like a natural mating posture for a hen cockatiel. The fact that she exhibits breeding behavior doesn’t mean that she has to have a mate. It simply means that she is having a natural biological reaction to her environment.
There are many aspects of a female cockatiel’s life that can stimulate breeding behavior. This includes an increase in light, heat and humidity. There can also be something in or near your cockatiel’s bird cage that seems like a nest or a nest cavity. It is normal for a cockatiel hen to behave in this manner from time to time.
However, do whatever you can to keep this "broody” behavior to a minimum. The reason is that if a companion cockatiel hen begins to lay eggs, it may lead to excessive egg laying, which can lead to calcium deficiencies, egg binding and other serious health problems.
Excerpt from the Popular Birds Series magabook Cockatiels with permission from its publisher, BowTie magazines, a division of BowTie Inc. Purchase Cockatiels here.