From the Super Mario Wiki
A Birdie is a score that can easily be achieved in the Mario Golf series. A Birdie is a score equal to one below Par. Birdies can be achieved on all holes and any character can achieve them. In Mario Golf: Toadstool Tour and Mario Golf: Advance Tour, Birdie Best Badges are blue, wheras in Mario Golf: World Tour, they're Silver. In Mario Golf (Nintendo 64) and Mario Golf (Game Boy Color), every time characters get a birdie in a Tournament they will get one Birdie Badge. After getting 50 of them, players unlock Maple in Mario Golf.