The black swan (Cygnus atratus) is a native of Australia. Imported birds, brought to private parks, gardens and ponds outside of Australia, sometimes escape or are set free and live wild. In Britain in 2001, there were at least nine breeding pairs of black swans reported.
Black swans differ from other swans not only in color but also in behavior. They nest on large mounds that they build in the middle of shallow water, reusing the same nest year after year. Unlike many other water birds, black swans do not migrate, instead spending their entire lives in the area where they were hatched.
The black swans living in Leiden, The Netherlands, are found mostly on the moat surrounding the city and close to a petting zoo in the Merenwijk neighborhood. The birds breed in the Dutch winter, because their biological clocks are set to Australian time.
Sources:
Een zwarte zwaan: gewoon en toch bijzonder
Black Swan on Wikipedia
Non-native birds breeding in the United Kingdom in 2001
