Warren Buffett’s company added to two of its sizeable new investments in major airlines during the first quarter and eliminated a stake in Twenty-First Century Fox Inc.

Berkshire Hathaway Inc. filed an update on the U.S. stocks it held at the end of March with the Securities and Exchange Commission on Monday.

Buffett had already disclosed in recent interviews that Berkshire sold off one-third of its 81 million IBM shares and bought about 133 million Apple shares.

The filing showed Berkshire added to its investments in American Airlines Group Inc. and Southwest Airlines Co. in the quarter. Its United Continental Holdings Inc. stake was unchanged, and its Delta Air Lines Inc. stake was reduced to 55 million from 60 million shares.

Berkshire held 49.3 million American shares at the end of March, up from 45.5 million. Its Southwest stake grew to 47.7 million from 43.2 million.

Buffett first disclosed his airline investments last fall, surprising many because he had long been critical of the industry. He has invested in all the major airlines now because he believes the industry is much healthier now and flying with planes at least 80 percent full most of the time.

Berkshire sold off nearly 9 million shares in Twenty-First Century Fox, the parent company of the 20th Century Fox movie studio and Fox News Channel, which has been hit by allegations of discrimination and harassment.

Berkshire officials don’t generally comment on the quarterly filings or the reasons for their portfolio moves.

Investors, however, like to look at Berkshire’s investments because of Buffett’s remarkably successful track record, although the filing doesn’t make clear who made all the investments. Buffett handles the biggest investments in Berkshire’s portfolio, such as Coca-Cola, Wells Fargo and IBM. He has said that investments of less than $1 billion are likely to be the work of Berkshire’s two other investment managers.

Besides investments, Berkshire owns more than 90 companies, including insurance, retail, manufacturing, railroad and candy companies.

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Photo Credit: The man who once said there was no faster way to lose money than investing in airlines, Berkshire Hathaway chairman and CEO Warren Buffett, speaks during an interview with Liz Claman of the Fox Business Network, in Omaha, Nebraska. Nati Harnik / Associated Press