Definition of "yardbird"

The working convict is a rare exception, sometimes envied because his time is occupied, sometimes derided for his deviance from the yardbird norm.

1985, John P. Conrad, “Charting a Course for Imprisonment Policy,”, in Annals of the American Academy of Political and Social Science, volume 478, page 126

As the Marines expanded to war strength, Lou Diamond was the ideal liaison between crusty old-timers and impressionable recruits. He taught quick action by threats of yardbird detail.

1943 July 5, “In the Rough”, in Time