Black swans, it turns out, are not that rare a bird, after all. In Australia and New Zealand, there are hundreds of thousands of them. Imported by private collectors, black swans have also spread to the wild in the United Kingdom and are bullying their white (also called "mute") swan cousins for territory.
In the investment context, "black swan events" may also be less rare than formerly thought. The concept, popularized by New York University professor — and former derivatives trader — Nicholas Nassim Taleb, is intended to describe unexpected events that have very big consequences. They don't have to be negative, but they do have a profound effect on the economies and markets they disrupt.