Birdo first appeared as the exit guardian in SMB2, guarding Crystal Balls and Keys while spitting eggs at intruders. Although it might sound odd to those unfamiliar with Mario games, there is some controversy among Mario fans regarding the sexual orientation of this pink dinosaur. The American SMB2 manual states that Birdo "wants to be a girl" and "would rather be called Birdetta." As Birdo began to appear in subsequent Mario games, Nintendo of America began referring to Birdo as "she," perhaps hoping to end further speculation.

In Mario Tennis, Yoshi and Birdo appear as a quasi-romantic couple in promotional artwork, a move that caused some confusion among Mario fans unfamiliar with Birdo's new orientation.

In Japan, Birdo has always been female, and is named "Catherine." Why NOA chose to initially give the character such an odd personality remains unclear.