Green’s Dictionary of Slang

1. (US, Southern) a Northern, Unionist soldier [the colour of the uniform].

1861		R.H. Newell Orpehus C. Kerr I 199: I reached the aside of the Commander of the Accomac [...] ‘How are you, my blue-bird’ [DA].
1867		J.M. Crawford Mosby and His Men 129: John Munson and Walter Whaley brought in two bluebirds, one walking and the other riding [DA].

2. (US) a police officer [the colour of the uniform].

1996		Eble Sl. and Sociability 83: There are 8 terms for ‘policeman’, including bluebird, Irvin, and skull-buster.
2005		J. Stahl I, Fatty 235: The Tenderloin bluebirds stepped back to their positions.

3. (Aus./US) a police car, a police wagon [the colour of the Buicks and later Fords that fulfilled the role].

1976		G.V. Higgins Judgment of Deke Hunter 43: Six years in a bluebird, hiding in the bushes and waiting for dentists to go by me too fast in their Jaguars.
1982		Verbatim VIII n.p.: bluebird, the police paddy-wagon.
2000		L. Shapiro Special police Vehicles 17/2: A pickup truck and trailer for the mounted unit, minivans for the forensic technicians, a Blue Bird bus for command purposes.

4. (drugs) a blue capsule of sodium amytal [packaging].

1965		K. Marlowe Mr Madam (1967) 20: He was then buying happy pills by the sack [...] Bluebirds of Happiness.
1977		L. Young et al. Recreational Drugs.
1981		D.E. Miller Bk of Jargon 327: Amytal. (amies, blues, bluebirds, blue devils, blue heavens).
2001		ONDCP Street Terms 3: Blue birds — Depressants.