In the nineteenth and early twentieth century, European visitors to Beijing commented on the eerie whistling music that sometimes could be heard as they passed down streets and alleyways. These noises accompanied the flight of flock of pigeons, and came from an ancient Chinese tradition of the pigeon whistle. In China, as in many other parts of the world, people have long kept pigeons as pets, training them for racing, messenger birds, homing pigeons and other practical and entertaining roles.
In Beijing, this domestic role of the pigeon is taken further; young pigeons are equipped with small bamboo or gourd flutes, tied to their tail feathers with wire. The flutes are very light, and do not impede the pigeon's flight, allowing the bird to join the flocks of other pigeons in the city. When they fly, air is forced into the mouthpiece of the flute and emits a high note, which quavers in the air as the pigeon moves. When several pigeon whistles come together, they can produce a complimentary effect, although pigeons are not generally sufficiently organised to produce coherent music.
Chinese tradition holds that these whistles aid pigeon flocks in co-ordination, and drive away predators, although there is little direct evidence of this effect, and pigeons elsewhere seem to cope effectively with these issues without artificial aids. It is more likely that this attractive effect appeals to the people who co-habit with the pigeons, rather than the birds themselves. The practice of attaching whistles to pigeons has largely died out in modern Beijing, the music lost perhaps under the noise of the city, although there are a few elderly practitioners who still make these whistles and send their birds out into the Beijing sky.