IIRC the birds were named after mythical deities like Rao. Since the destruction of Krypton the religious guild has become more active and I guess revived the worship of the Flamebird and Nightwing gods along with Rao. For some reason Thara has had vision of Flamebird all her life and the guild found that out and performed some ritual on her that made her realize she is the Flamebird's avatar. Then some psychic connection between her and Chris sparked, which made them realize he is Nightwing.