cockatiel gendering

So I found this forum and I thought I would register because I'm looking for one answer. I have a cockatiel who is maybe a year/less than a year old. We got it at the pet shop and it was said that it was a female. At first she didn't eat, she'd sleep all day, never sung etc. until we found...

I have two cockatiels, and they sometimes rub their heads together do only partners do couples do that? I have a male, and one that i'm not sure, it has speckles on the back of its tail feathers, but also likes to imitate my male cockatiel when most people say female cockatiels can't sing. It's...