A stool pigeon is someone who betrays a group that they belong to. An informant for an organized crime syndicate, or a traitor to a country would be considered stool pigeons. These people are often looked upon quite poorly as you may guess, although the origin of the term doesn't quite seem to apply.
The term stool pigeon comes from a time when passenger pigeons still existed. Hunters would tie a passenger pigeon to a stool and the cries of the bird would attract flocks of passenger pigeons. The hunter would then shoot these birds by the thousands; they did this until the bird was extinct. It is said that there was once over 5 billion of these birds in North America, the last passenger pigeon died in the Cincinnati Zoo on September 1, 1914, it's name was Martha.
The term 'stool pigeon' doesn't quite apply because the pigeons tied to the stools were unwilling participants, whereas people who are referred to as 'stool pigeons' willingly betray the group they belong to.
The passenger pigeon unfortunately is only one of a great many species the human race has needlessly made extinct.
