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There used to be a pair of Black Swans on the lake at York University - a gift from Australia - but vandals smashed their nest and all their eggs. The swans were so distraught they vanished and never came back.
There have been lots of escaped Black Swans, and they are slowly becoming established in the UK as with Little Owls and Mandarin ducks some have quite strong feral and self sustaining populations.
These are actually unwanted visitors to the swannery. Black swans tend to be aggressive to the white swan and hang about because of the food and nesting around the site.