Hartz Mountain Corporation was founded by two guys who emigrated from Germany to the United States in 1926 with five thousand (!!!) singing canaries. - paraphrased from Wikipedia
The Harz Roller is a breed of domestic canary bred in the Upper Harz mountains of Germany. The birds were bred in the middle of the 19th century and achieved European-wide fame.
With patience a breed of canary was produced that had a very pleasant, melodious song, full of variety and delivered with an apparently closed beak. In the second half of the 19th century the breeding and sale of these canaries boomed.
The birds were used as a warning system to indicate the presence of poisonous gases such as carbon monoxide and low levels of oxygen in the surrounding air. Canaries were especially good for this purpose as, unlike finches, doves and mice, they reacted very quickly to carbon monoxide. While a mouse would not have a noticeable reaction until after up to 70 minutes to a carbon monoxide concentration of 0.77 % in the air, a canary will fall off its perch after as little as 2.5 minutes from a concentration of 0.29 %. For this reason canaries were not only used in normal operations, but especially to protect rescue units in the case of an accident. - paraphrased from WikipediaI looked for a YouTube video of a singing canary, but I didn't find one I liked. The ones I listened to didn't sound very melodious or pleasant, but maybe that's just a limitation of YouTube.