 During the establishment of formal pigeon post services, the registration of all birds was introduced.At the same time, in order to hinder the efficiency of the systems of foreign countries, difficulties were placed in the way of the importation of their birds for training, and in a few cases falcons were specially trained to interrupt the service war-time, the Germans having set the example by employing hawks against the Paris pigeons in 1870–71.No satisfactory method of protecting the weaker birds seems to have been developed, though the Chinese formerly provided their pigeons with whistles and bells to scare away birds of prey.