Birdo has always been a complex character, especially for the Mario series. In Japan, they're known as 'キャサリン', which loosely translates to 'Catherine' in English, and was referred to in the instruction manual of Yume Kōjō: Doki Doki Panic with male pronouns. Similarly, in the move to the West and under the revised branding and coding of Super Mario Bros. 2, Birdo was described in the manual with the line "He thinks he is a girl".
This has subsequently been retconned outside of Japan, and for the years following Birdo was instead referred to with female pronouns. With the release of Super Mario Party, however, Birdo's gender has been made more ambiguous, and things only get more complicated when you take the region you come from into account.
If you navigate towards the 'Advice' section of the Party Pad, you can find some advice on the in-game Stickers you can unlock, and it mentions how Birdo can help you grab more should you wish. In the United States and Canada, nothing has changed and Birdo is still referred to with female pronouns. In the United Kingdom and the Netherlands, things are turned upside-down and Birdo is described with male pronouns.
Japan, as well as Spain, take a wholly different approach, instead skirting around using any gendered pronouns at all, thus keeping any reference to Birdo's gender intentionally unclear. We've only been able to compile this information thanks to the help of fans like all of you getting in touch on Twitter, so thank you! We'll be sure to add more regions into this article as we learn more.
It's extremely interesting to see a shift like this from Nintendo, and one that will no doubt make for an interesting debate in the comments. Please be mindful of everyone else's views, and be respectful of other people's opinions as much as possible. Cheers!