(Adds details from filing, byline)
By Jonathan Stempel
NEW YORK, May 15 (Reuters) - Warren Buffett's Berkshire Hathaway Inc on Monday said it added to its sizable investments in American Airlines Group Inc and Southwest Airlines Co, and shed its stake in Twenty-First Century Fox Inc.
In a regulatory filing, Berkshire said that in the first quarter its American stake grew 8 percent to 49.3 million shares worth $2.08 billion, while its Southwest stake grew 10 percent to 47.7 million shares worth $2.57 billion.
Berkshire also shed 8 percent of its holdings in Delta Air Lines Inc, ending March with 55 million shares worth $2.53 billion.
Buffett became one of the largest investors in American, Delta, Southwest and United Continental Holdings Inc last year, reversing his disdain for an industry he once called a "death trap" for investors.
He told shareholders at Berkshire's annual meeting on May 6 that airlines' business model had improved, with fewer unsold seats and an ability to charge for checked bags, seat choices and other amenities that were once free.
Berkshire's sale of its $251 million Fox stake came several months after the parent of Fox News became embroiled in a sexual harassment scandal, but before news surfaced that the network and top-rated host Bill O'Reilly paid about $13 million to settle harassment claims by five women.
O'Reilly left Fox last month.
It is unclear why Berkshire sold Fox or who made the investment, which began in 2014.
Smaller equity investments at Berkshire are normally made by Buffett's deputies Todd Combs and Ted Weschler, who together recently invested about $21 billion.
Buffett's assistant did not immediately respond to a request for comment.
Berkshire's stock holdings are closely scrutinized by investors hoping to learn what has won or lost Buffett's favor.
The Omaha, Nebraska-based conglomerate also owns some 90 businesses including the BNSF railroad, Geico car insurance and Dairy Queen ice cream.
In Monday's filing, Berkshire also said it owned 129.4 million Apple Inc shares worth $18.6 billion as of March 31, fewer than the 133 million shares that Buffett had said Berkshire owned a month earlier.
It is unclear what accounted for the discrepancy.
Berkshire also announced larger stakes in Bank of New York Mellon Co and satellite radio operator Sirius XM Holdings Inc, and reduced stakes in International Business Machines Corp and commercial vehicle parts supplier Wabco Holdings Inc.
Buffett told CNBC on May 4 he has sold close to one-third of what had been an 81.2 million share stake in IBM. (Reporting by Jonathan Stempel in New York; Editing by Jonathan Oatis and Phil Berlowitz)