This ought to be enough information for anyone on Ostro/Birdo...from my own personal collection, I bring you, SCANS
1988 NES Game Manual (Super Mario Bros. 2)
1991 Super Mario World Player's Guide "Mario Mania"
1994 Super Mario All-Stars Player's Guide
2001 Super Mario Advance: Super Mario Bros. 2 Manual
Also, if you notice, the later ones don't touch upon Gender. I think the Japanese idea behind the character was created, and later complained about by some parents, so they try to cast it off as just a "creature".