Game of Thrones is a staple of Time Warner's HBO. Image: HBO.

Hedge fund manager David Einhorn has a remarkable track record of success. Although his fund, Greenlight Capital, had a difficult year in 2015, it has fantastically outperformed the broader market over its 20 year history. For this reason, Einhorn's holdings (disclosed in Greenlight's regular 13F filings) are closely watched by many investors.

Media giant Time Warner (NYSE:TWX.DL), the owner of CNN, HBO, and Warner Bros., has become a favorite of Greenlight. As of the end of last year, it was the fund's third-largest holding. What does Einhorn see in Time Warner, and should investors care?

Buying up Time Warner
Greenlight first acquired Time Warner shares during the fourth quarter of 2014. It immediately made it a fairly sizable investment, representing just over 4% of its holdings. It bought more Time Warner stock in the quarters that followed, most notably in the fourth quarter of 2015. In February, Greenlight filed its most recent 13F. It showed that Time Warner accounted for about 7.6% of its portfolio.

It's possible that Greenlight may have dumped all of its Time Warner stock since the turn of the year, or expanded its investment significantly. Unfortunately, 13Fs are filed only four times a year, and are delayed by 45 days. They represent a fund's positions on a given date at the end of a quarter. Still, they give investors a sense of which stocks a fund manager is interested in.

Although it's certainly possible, It would be surprising if Greenlight dumped its Time Warner stake last quarter. Only two other stocks represented a larger chunk of Greenlight's portfolio: Apple (NASDAQ:AAPL) (at around 12%) and General Motors (at around 8.7%). Greenlight isn't Berkshire Hathaway -- it buys new stocks and sells out of companies entirely on a semi-regular basis -- but it often invests in individual companies for many years. Most notably, Apple has been a core Greenlight holding since 2010, and the fund has held a stake in General Motors for most of the last four years (it exited the auto-maker in 2014, but bought back in early last year).

Einhorn hasn't said much
Einhorn is known for his shareholder activism, and frequently makes regular media appearances to promote his holdings. In 2013, for example, he waged an active campaign to encourage Apple's management to return its mounting cash pile to shareholders. Although his unusual proposal -- dubbed "iPrefs" -- was ignored, Apple expanded its dividend and share buyback program in the quarters that followed.

He's been considerably less vocal about Time Warner. Indeed, the only public comments Einhorn has made come from his January 2015 letter to investors. In it (via Valuewalk), Einhorn announced the stake, and offered some explanation: In the wake of its failed merger, Time Warner's management would be heavily incentivized to reward shareholders.

"...we established a new position in Time Warner...In July, Rupert Murdoch launched an opportunistic take-over bid and the shares soared. The TWX board refused to engage, Murdoch walked away and the stock returned to pre-takeover levels. We purchased a position in TWX at an average price of $72.72, believing that management would have to respond forcefully to fend off another advance. In particular, we believed that TWX had an opportunity to more aggressively monetize HBO and to reduce costs across the entire company. Management subsequently announced that HBO would be offered as a stand-alone streaming product in the U.S., along with various other initiatives that have led to an increase in earnings estimates and a rally in the shares, which ended the year at $85.42."

Currently, Time Warner shares trade for around $73 -- down more than 14% in the last 12 months. Media stocks in general have had a tough time, but Time Warner's disappointing performance cannot be ignored.

When Greenlight files its next 13F in May, I would expect a more all-encompassing assessment of the investment, and perhaps a shift in strategy. Other funds have begun amassing stakes in the media giant, and there are widespread rumors of possible shareholder activism. If Einhorn is still holding Time Warner shares, he may have some ideas to help get the stock back on track.