You become the 'Birdo' by virtue of being last in a Mario Kart league system, however you can instantly become 'The Birdo' for finished last in all 8 races in an 8 race single event.
Such as Booty, car, buildings...
Place: Q: Did you just get back from the office? A: You mean the Birdo? Response: Yeah
Thing: Q: Can you pass me that Birdo? A: This birdo? Response: For shizzle\
Man that bitch be acting Birdoish. Hellez yea.