One stroke under par on a hole — a 3 on a par-4, for example. The name comes from 1900s American slang, where "bird" meant anything excellent. Birdies matter because they're the currency of scoring: pars keep you steady, but birdies are what actually move you down the leaderboard. For most amateurs even one birdie makes the round; tour pros average about 3–4 per round.