In golf, scores that are one stroke under par (the expected number of hits for a hole); or small birds in general.
From 'birdie,' which came from American golf slang in the 1890s. A player calling a good shot 'a bird' evolved into 'birdie' for one-under-par, likely influenced by the phrase 'a bird in the hand.'
Golf slang created 'birdie,' 'eagle,' and 'albatross' for increasingly good scores—it's a whole bird-themed vocabulary hidden in a sport! This shows how communities create their own languages within languages.
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