My cockatiel tries to bite my male cockatiel when he sings
I have had a male cockatiel for a little under one year now, he is finishing up his first molt. About 3 months ago, I got another cockatiel who is under a year old and I think is a female (still waiting for DNA testing). Sometimes, my boy will walk up to her with heart wings and start singing in her face; he will also, at times, lean towards her head like he wants to preen her while singing. When he tries to preen her head, she snaps at him and they smash beaks. Why is this happening? Is there anything I can do to encourage at least tolerance between them? They are currently kept in separate cages. He is still very sweet and affectionate towards me, she is taking a little bit more time to warm up than he did. Any help is appreciated, thanks!