The score you make on a golf hole is determined by the amount of strokes you need from the start of the hole till the ball is in the cup. ‘Par’ is the number of pre-determined strokes a professional golfer should require to finish a hole. Depending on the length of a golf hole the ‘par’ is 3, 4 or 5 strokes. If you finish the hole in one stroke less then par, so for instance in just 2 strokes on a par 3 hole, that score is traditionally called a ‘birdie’.
This expression originated in 1899, at the Atlantic City Country Club in the United States. One day three golfers – George Crump, William Poultney Smith and his brother Ab Smith – were playing together when Crump hit his second shot only inches from the cup on a par-four hole after his first shot had struck a bird in flight. Simultaneously, the Smith brothers exclaimed that Crump’s shot was “a bird.” Crump’s short putt left him one-under-par for the hole, and from that day the three of them referred to such a score as a “birdie.” This name caught on and got widespread over the years, now being the common name for a score one-under-par on a hole.