I continue to maintain the same position I've always held: There are multiple Birdos. Yes, the manual talks about hir as if seh was a character, but it talks about Ostro the same way, and there are plenty of times when you can see multiple Ostros at once. Enemies are always treated like individual characters, especially in Japan, where attitudes toward individualism and collectivism are significantly different than ours, but if you're going to get scientific about it, you have to accept that they're species. I'm sure there exists at least one transgendered Birdo somewhere, but there are also ones that are fully male and fully female.