Black swans.

You are going to start hearing about these birds a lot if the stock market keeps going lower because of the worldwide coronavirus scare.

No, the swans didn’t cause the virus.

The phrase “black swans” is a metaphor for an event that happens as a surprise and has a major effect.

The coronavirus, in other words, is an event — a black swan — that investors didn’t expect when they bid up stock prices to levels that reached bubblicious proportions.

The only problem with that metaphor is that black swans really aren’t all that rare in nature. And potential black swans certainly aren’t in short supply these days, with international tensions, political battles, monetary surprises and the like all waiting their turn to shock the financial markets.

Last Friday’s 600-plus-point drop in the Dow Jones industrial average is one example of how badly a black swan can damage investor confidence.

The trouble is, the coronavirus shouldn’t really have surprised investors on Friday. It had been spreading for days, causing companies to cut back on their deals in the Far East and leaving doubts about the overall damaging effect on the world economy.

What helped Wall Street ignore the virus for a couple of days is the fact that end-of-the-month buying was able to push stock prices higher for a while.

Black swans like the coronavirus are the things that pop financial bubbles.

There’s one other thing going on that can damage bubbles, but there is no cute waterfowl way of describing it.

This stock market bubble has mostly been caused by the Federal Reserve’s easy monetary policy. And we have already seen how much Wall Street hates the idea that the Fed inevitably will have to raise interest rates someday.

The Fed’s balance sheet could also be making Wall Street nervous. And that could be the blackest swan of all.

According to data from Bloomberg News, the Fed’s balance sheet has been flat to down since the middle of December. And if that means the Fed is not expanding credit as much as it had been, Wall Street will continue to swan dive.

We find out what effect the Christmas shopping season had on retailers at the end of the week when the Labor Department reports January employment data.

Other government numbers showed people spent a lot this Christmas.

Consumers said they were going to spend generously before the holiday season and they apparently did — with growth of around 3.4 percent over 2018’s season.

Why, then, are retailers still shutting down stores at a rapid pace?