Colorado mockingbird, n. — Green’s Dictionary of Slang

Colorado mockingbird n.

[the noise of its braying, the antithesis of a mellifluous bird]

(US) a donkey, an ass.

1968		R.F. Adams Western Words (2nd edn) 71/1: Colorado mockingbird A burro.
1978		Backpacker 28 Aug. 28: The desert canary, the Colorado mockingbird, was asserting his rights to an undisturbed meal.