Pigeon whistle is a municipal heritage of old Beijing. Called the echo from heaven, it's a cultural symbol of the Hutong houses. But, with the concrete jungle fast-encroaching the old neighborhoods, it's hard to hear this beautiful symphony. Ma Kewei is a whistle maker with almost 40 years of experience. For him, making whistles and playing with pigeons are a big part of his life since 1982. The whistles are made with materials like gourds and bamboo. Ma says the whistles should be lightweight for the pigeons to carry, and he usually plants gourds in his own yard. Mu Ruixin is Ma's friend and also a pigeon fancier. He knows how to attach whistles to the birds' tail feathers gently to create a flying orchestra. After that, pigeons should be placed in their homes for a while to get used to the weight on their tails.