It’s damn hard to convince children that Birdo was (or is still) a male character.
You don't make convincing children of your original male-ness easy, Birdo.
“Birdo is a boy.”
“But she looks like a girl.”
“But he isn’t.”
“But she’s wearing a bow and is pink. She’s a girl.”
“Listen to the voice in Mario Kart. That’s a throaty whisper a woman can’t make.”
“But she looks like a girl.”
“But he’s a boy.”
“Is not!”
And so on.
You may wonder why I’d get into this issue with kids, but it’s important to tell children the truth, right? It’s also a step towards understanding a wider world, even as far as knowing that a number of Asian cultures have long established transgender subcultures, to the point that transgender aren’t as big a deal there as they can be in Western society.
Next up for the kids: the truth about Santa Claus.
World Class Villains 
Need to find an original copy of the Mario 2 manual and scan the character page. It’s pretty clear there. =)
Your work has been done for you! http://en.wikipedia.org/wiki/File:Super_Mario_Bros_2_-_Birdo–article_image.jpg
But Nintendo has retconned things from time to time too.