Pigeon whistles, called geling 鴿鈴 or geshao 鴿哨, are lightweight (8 grams or less) whistles tied to the backs of pigeons so that they produce a musical sound when the birds fly. This has been going on in China for a thousand years, first by the military as a means of communication, then by people who kept and trained pigeons as a hobby. Pigeon whistles are made from bamboo or dried gourds, and these can be combined to produce multiple sounds. They are attached to the bird's four main tail feathers, and do not hinder or injure the pigeon. There are strict traditions for making the whistles, so that multiple birds will sound harmonious when flying.
The sight and sounds of pigeons carrying whistles are likened to "old Beijing," because the art is dying out. Keeping pigeons declined when China's architecture turned to skyscrapers and modern-looking homes, which left little room for nests. There are still a few experts in the craft of making the whistles, but they are entering the realm of collectibles. -via Present & Correct