One fewer is the good news
A birdie is a score of one stroke under par on a single golf hole. If the hole is a par four and you finish in three strokes, that is a birdie.
The PGA glossary describes par as the score an accomplished player is expected to make. It is the reference number that gives birdie its meaning.
Three is not always a birdie
Imagine two friends both finish a hole in three strokes. One played a par four; the other played a par three.
The first has a birdie. The second has made par. Same number of strokes, different result against the hole’s reference score. Cambridge Dictionary makes that one-below-par relationship explicit.
The tiny score check
For a birdie, subtract one from the hole’s par: two strokes on a par three, three on a par four, or four on a par five.
Golf gives nearby scores other names, too. In the PGA glossary, a bogey is one over par and an eagle is two under. A birdie sits one stroke below par, without needing a feather in sight.
A small number with context
The useful question is not simply how many strokes someone took. Ask what par was for that hole, then compare the two numbers.
For another golf surprise, find out why the ball has dimples. Or compare another sporting score with a football hat-trick.