The latest 13F season is here, when many money managers issue required reports on their holdings. It can be worthwhile to pay attention, as you might get an investment idea or two by seeing what some major investors have been buying and selling.

For example, consider Appaloosa Management, founded by investing giant David Tepper and known for investing in the debt of companies in distress. Tepper's investing history includes debt and stock in companies such as Enron and Worldcom. He made billions on bank stocks in 2009 after they had imploded and before they recovered. More recently, he invested in many housing-related companies. In a letter to shareholders last year, Tepper noted that had one invested $1 million in his hedge fund in 1993, it would have grown to $149 million over the past 20 years. Investing in the S&P 500 instead would have left you with $5.3 million. Tepper's performance reflects an average annual net gain of 28%. Wow.

Appaloosa Management's latest 13F report shows that it reduced its positions in Delta Air Lines (NYSE:DAL), United Continental Holdings (NYSE:UAL), and J.C. Penney Company (NYSE:JCP).

Delta Air Lines' stock more than doubled over the past year and averaged more than 50% growth annually over the past five. In the past year, the airline industry has been hammered by bad weather, but Delta has still posted increases in its unit revenue. (Consider that just in 2014, both Delta and United Continental have had to cancel roughly 20,000 or more flights!) Clearly, the company is doing something right. It has become a cash flow leader in its industry, for example, expecting to top $5 billion in 2014. Still, bears worry that its momentum may not be sustainable.

United Continental, meanwhile, has also seen its stock surge, and it carries a strong buy rating from analysts at Zacks Equity Research, who like its cost-cutting and expanding network (which includes new nonstop service between Los Angeles and Melbourne, Australia, and plans to connect San Francisco and Tokyo). United is forecasting weak results in its first quarter due to weather, but some see the company routinely making excuses.

J.C. Penney, meanwhile, has been struggling so much lately that some have wondered whether it will end up seeking bankruptcy protection in the next few years. Its fourth-quarter report, though, stunned skeptics, revealing some growing revenue and a smaller-than-expected loss. Website revenue grew by double digits, too. Some remain skeptical, however, and see sustainable profitability as unlikely.

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Selena Maranjian, whom you can follow on Twitterhas no position in any stocks mentioned, and neither does The Motley Fool. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.