Although in Japan, Birdo has always been a male character going by the alias "Catherine", there has been some disparity of this fact in Western releases. In the manual for Super Mario Bros. 2, it was disclosed Birdo was in fact a male who wished they were female, going so far as to request people call them "Birdetta". Nintendo has since depicted Birdo wearing a diamond ring to complement the existing bright red bow, and abstracted any dialogue by merely using what can be described as a 'honk'.
Earlier in the series of Mario sports games, Birdo was listed as female, however most recently the fact has been shied away from, and no particular descriptors have been used either way. Furthermore, in Super Smash Bros. Brawl, the text description alongside the Birdo trophy makes claims of an 'indeterminate gender'.
In the 2008 Japan-only Nintendo Wii game Captain Rainbow Birdo is voiced by a distinguished male voice actor.