"The birds are singing."
This is an irregular plural ending. There are quite a number of these in Hebrew. The word ציפור is female, despite the fact that it doesn't end in ה. Strangely enough, the plural of ציפור is ציפורים, which makes it doubly confusing. So ציפור שרה, and ציפורים שרות.
This word is ancient and is feminine in classical Hebrew and in Aramaic. However, that the word does not present as feminine has always caused problems. It's unclear to me whether the -im ending means it presents as masc or is actually masc in the plural. The DCH (Dictionary of Classical Hebrew, 7.148), for instance, states that the noun is masc. and fem., which may be the way of that dictionary's editor (David Clines) of noting that the lack of feminine presentation caused some ancient authors to get confused, but I have not looked into the matter extensively. The Comprehensive Aramaic Lexicon project also assigns the Aramaic form as masc. and fem.: http://cal.huc.edu/ Long story short, if you are confused by its morphology you stand in a long line. Cf. Dalit Rom-Shiloni, “Birds in the Bible,” Blackbird Project: Educational Cooperation between Arab and Jewish Students in Birding (ed. Yossi Leshem and Anat Levi; Tel Aviv, 2009), 72–83.
What a mess. There is no specific form for a male bird or a group of them and the female form does not look female. Only the accompanying verb and adjective clarify this point. Interesting that this confusion happened with birds. Were there ever two ancient noun forms specifically for female and male birds? Imagine underlying all these linguistic issues was really just a reluctant realisation all these strikingly beautiful singing birds are actually the males. Who knows, but at least this story helps to master the בלגן.
There is no masculine form. If you refer to masculine birds you choose the masculine form of the accompanying adjective, verb etc.
fe: הציפסרים הכחולים שרים = The blue male birds sing.