One of Warren Buffett's signature catch phrases is "be greedy when others are fearful." Berkshire Hathaway (NYSE:BRK.A)(NYSE:BRK.B) put Buffett's philosophy to work during last week's market plunge, boosting its stake in Delta Air Lines (NYSE:DAL) following the airline stock's 20%-plus fall.

Airline stocks have been among the hardest hit by the COVID-19-related sell-off, with investors worried that concerns about the novel coronavirus will eat into travel demand heading into the peak U.S. summer vacation season. Many large corporations have also suspended non-essential travel.

A Delta A321 parked at an airport.

Image source: Delta Air Lines.

In a regulatory filing, Berkshire said it bought more than 976,000 shares of Delta last week for about $45.3 million, for an average cost of about $46.40 per share. That's not far from Delta's 52-week low of $44.33 hit last week. The airline's shares have traded as high as $63.44 per share in the last year.

Berkshire holds a basket of airline stocks, but the company's disclosure rules for Delta are different because Berkshire owns more than 10% of Delta's shares outstanding. Investors will find out at the end of the first quarter if Berkshire Hathaway was buying other stocks during the market decline.

Delta had been a big winner for Berkshire Hathaway prior to the coronavirus-related sell-off. It first began buying in 2016, and had a paper gain of more than $4 billion, or nearly 30%, as of last November.

With the latest purchase, Berkshire now owns more than 71.9 million shares of Delta, or 11.1% of the shares outstanding.