Birdo's voice pretty much confirms that it is male.
We're working on a Birdo special that will cover this, but in Japan, Birdo is still a male who thinks he's female. At the beginning, one of the subtitles says "Atashi wa Catherine" (I am Catherine), but
atashi is primarily used by females. Also, the subtitles are pretty interesting because the spoken Japanese sounds like it's scrambled and does not match the subtitles. For example, every time "E?" (pronounced "Eh?") comes up the voice says "Torima?"