What is my cockatiel doing?

Question
Hi there.  I bought a cockatiel (my first bird) in August of 2007.  Just recently Gracie has started to sit on her top perch, stick her tail feathers in the air with her head down (giving you a clear shot there) and chirping the tiniest little chirps.  At first I thought I was hearing a mouse, then realized it was her!  I'm wondering... do birds go into a kind of "heat" like cats and dogs?  Is she approaching puberty?  And I'm wondering if laying eggs comes next?

Thanks so much!
Suzanne

Answer
hi. thanks for your question.
cockatiels usually reach the age of sexual maturity by about 1 1/2 to 2 years of age. if you purchased her in August, that means she is at least about 10 months of age (obviously depending on how old the breeder/pet shop told you she was when you purchased her). so it is quite possible that she is getting closer to attaining sexual maturity.  
captive female birds are different from female mammals in that there is not a "heat" period where she exhibits sexualized behavior and a "readiness" to mate. in fact, captive birds can breed all year round and are able to breed year round. so you would not see her exhibiting that periodic highly sexualized behavior that is abound in cats/dogs "in heat."
a behavior that female birds of a sexually mature age WILL do is masturbate. male birds do the same thing although it is in a different manner.  female birds will usually attempt to 'back' into things and try rubbing the top of their tail (the rump area) against anything in sight....this includes perches, toys, cage bars, and sometimes unfortunately, people.  each bird is different however what you are describing sounds like possible masturbatory behavior...anything involving raising/rubbing the tail and especially putting her head down in order to help facilitate this position. the chirps are probably just part of that behavior as well.
this is not cause for concern though it can be sort of embarrassing.  since she is captive, she is trying to self-stimulate, especially if she is just coming in to her sexual maturity. you may observe this behavior off and on for most of her adult life.
whether or not eggs end up coming next is debatable...it may not occur right away.   usually solitary female birds will lay eggs when they come in to sexual maturity. since it sounds like she is getting there, you may expect a clutch of eggs sometime in the near future...it doesn't necessarily coincide with the onset of masturbatory behavior but chances are it will happen within the next year or so.  
as far as the behavior goes, you are correct in that she will be going through an adolescent/puberty phase and it is best to just let in run its course and let her "do her thing".  
good luck and i hope this helps!  alicia