In poker, a person who watches a game from the “rail”. It’s a term which is derived from live poker because in live cardrooms there is a physical rail that separates the players/tables from the observers. These “railbirds” could be watching a friend at the table or waiting to join a game.
The term is also used in online poker games. In this case, it refers to observers who are watching an online cash game or tournament and chatting in the online chat box. High stakes online cash games on Full Tilt and Poker Stars have always attracted lots of online railbirds.