"If you wish to teach the Birds airs, or artificial notes of any kind, they must hear nothing that can in any way distract their attention. Every time you enter the room, the oftener the better, and especially when you feed them, whistle, or play on a flute or flageolet, the tune you wish them to learn. Whistle or play that, and no other. Repeat, and repeat, and repeat, until they can pipe it correctly."
People would capture wild birds of all kinds and either teach them human songs or raise them with nightingales so they would have a nightingale's song. Some people even sold birds that had been trained this way. But getting food to feed the birds could be troublesome:
"Take some meat, or fish, or a dead cat, rat, or dog, and hang it in a shady place until it is well fly-blown or maggoty. Then place it in a large box half full of earth, and cover. In the course of a week or ten days the maggots will bury themselves in the earth, and may be dug up, if the box is kept in a cold place, at leisure."
I wonder if the cats and dogs just happened to be dead, or were killed specifically for this purpose.
--From Manual of Cage-birds, British & Foreign--