Green’s Dictionary of Slang

Rocky Mountain canary (bird) n.

a donkey, an ass.

1883		Catochin Clarion (Mechanicstown, MD) 23 Aug. 1/4: Ore is packed in ore sacks on ‘burros’ (Jacks or Rocky Mountain Canaries some call them here).
1893		Eve. World (NY) 7 July 5/2: The actress received a present of a burro [...] alias a Rocky Mountain canary bird.
1898		Southern Pacific Company Sunset 42: Trails so tortuous that even the surefooted Rocky Mountain ‘canary,’ the little pack burro, often plunges to his death.
1913		Wash. Herald (DC) 4 Aug. 9/1: The party will [...] get astride the ‘Rocky Mountain Canaries’ (burros) and ride through the South Cheyenne Canon.
1928		H.E. Collins Warpath and Cattle Trail 224: I have even been informed that my vocal efforts put every one else out of tune, by reason of resemblance to the famed ‘Rocky Mountain Canary.’.
1945		F. Cunningham Big Dan: The Story of a Colorful Railroader 270: He knew how Colorado transportation had progressed from the days of the Rocky Mountain ‘canary’ — the burro.
1962		H.W. Felton Sergeant O’Keefe and His Mule, Balaam 16: Other Rocky Mountain canaries since then had sung their songs on the great Peak, but none could equal Balaam, the greatest Rocky Mountain canary of them all.
1975		M.J, Kedro ‘The Rocky Mountain Canary: An Unsung Hero’ in Mountain and Plain Hist. Notes XII:1 on Homepage at Mindspring.com 🌐 His sportive annual pack race across Mosquito Pass aside, the Rocky Mountain Canary or Colorado Mocking Bird, so known for his hoarse and discordant bray, made important contributions to the settlement of the West. The burro held a special place in the heart of many a prospector.
1984		Compass of Sigma Gamma Epsilon 129: As no prospecting outfit is complete without its ‘Rocky Mountain Canary,’ no efforts were spared in looking for a burro.
1991		R. Moody Little Britches 26: Standing between the two big horses was a Rocky Mountain canary — a little donkey.