Concerns that cattle diseases might be carried across international boundaries have so far lacked documentation, but populations are being monitored and movements of banded birds are being tracked. Waterfowl Many species of exotic waterfowl have found their way into the wild through intentional introductions and by escaping from captivity. Many European and some Asiatic ducks, and even a few exotic geese, escape from private collections, especially during storms. Because the number of these individuals is small, their populations have not been monitored. The mute swan of Eurasia is the notable exception.
Small populations thrive in other localities.
Most were released to provide more hunting opportunities. Some states, such as Oregon, still have active introduction programs. Now they are one of the most noticeable birds in American cities and farming communities. Countless thousands are still reared annually by pigeon fanciers who use them for homing and racing competitions, and each year the feral population is supplemented by captive-reared individuals that fail to return home. Rock dove populations were ignored by scientists and bird watchers before the Breeding Bird Survey began in 1966 and were not reported on Christmas Bird Counts until 1974.