A general way to describe a group or groups of people hanging out at a public park (often skateparks). A birdgang partakes in the use of drugs, alcohol, or nicotine while doing close to no other activities other than loitering. Generally these people are ages 13-25, and do a good job of evading the police that constantly creep up on them. Of course the police want to stop underage drug use in public parks, but hardly ever catch them in the act.
"The birdgang at Gabriel park is ridiculous. Every time I go they are always smoking cigarettes and passing around a joint"