Due to contractual obligations with Warner Bros. the show runners are not allowed to use any other Hanna-Barbera characters in the show.
Birdgirl is a spin-off of the television series Harvey Birdman, Attorney at Law (2000), created by Michael Ouweleen and Erik Richter, which was a comedic spin-off of the original 1967 Birdman (1967) cartoon by Hanna-Barbera.
Spin-off to Harvey Birdman Attorney At law now focusing on Phil Ken Sebben's Daughter Judy Ken Sebben as CEO of Sebben & Sebben.