"A beautiful bird."
Well, both צִפּוֹרָה and צִפּוֹרִית are diminutive forms, ie. little bird, birdie. Zipporah was the name of Moses' wife, so it has become a first name for women too. If she likes it, you may call your darling a צִפּוֹרִית too. Therefore we do not have one word for female bird, but if you want to stress its sex, you may call it צִפּוֹר נְקֵבָה.