The shuttlecock used in badminton is also called a shuttle or a birdie. Apart from the synthetic shuttle, the professional shuttle is actually made of goose or duck feathers. This is one of the reasons why the shuttle is called a bird or birdie.
A facility that holds a professional badminton tournament comprises of multiple courts adjacent to each other. Multiple matches can be held simultaneously during a tournament. There is a chance when the shuttle of one court flings to another court. In such a case the umpire gives the call “bird on.”
When an umpire gives a call of bird on, then the ongoing point is immediately stopped. The point is then replayed. Alternatively, the umpire can also yell “let” instead of bird on.
A bird on is a very rare instance in professional badminton. While “bird on” specifically refers to a shuttle’s intrusion, the umbrella term for defining this instance is a let. According to Law 14.2.7 that details about “let’s” in badminton, not only a shuttle from another court, but any object that falls within the court is said to disrupt the flow of the game. Thus, we rarely here the term “bird on” and are more acquainted to a call of “let” in case of a hindrance.
Bird on is a common instance in local tournaments. However, since these tournaments also feature less experience chair umpires they might not be familiar with the call of “bird on.” This is why “let” has been popularized in such cases.