"Nintendo eventually established Birdo as female, first in Mario Tennis which depicted Birdo and Yoshi as boyfriend and girlfriend. However, several games portray her asmale, such as Captain Rainbow, or refer to the gender confusion without assigning one, such as in Super Smash Bros. Brawl."
I'm not so sure it's great to be birdo. Why? I'll give you a hint. That's not a mouth. Eggs come out of that thing. You know what that thing is? Yeah, you know what that big thing in the middle of her face is.
Actually, I think they retconned him/her into being a girl. Sort of how they retconned SMB3 into being just a play so they didn't have to explain why Bowser Jr. wasn't with Bowser's other kids. The Koopalings aren't actually Bowser's kids; the Toads who wrote the play just thought they were.