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 Citadel Advisors, founded and run by Kenneth Griffin. It's one of the biggest hedge fund companies around, with a reportable stock portfolio totaling $76.8 billion in value as of Dec. 31, 2013. According to the folks at, Griffin and his team use "a combination of advanced computer code, complicated financial algorithms and secrecy. Griffin was using quantitative, technology-based methods before many other firms had cell phones."

Citadel Advisors' latest 13F report shows that it boosted its positions in Bristol-Myers Squibb (BMY 1.02%) and MGM Resorts International (MGM -1.35%) significantly, while cutting its long position in Kodiak Oil and Gas Corporation (NYSE: KOG) by 57%.

Bristol-Myers Squibb's stock has surged almost 50% over the past year and has averaged  a solid 12% annually over the past 30. The company has been focusing more on specialized, niche drugs lately, and less on formulas for widespread conditions. It has new formulas in its pipeline, targeting cancers and viruses, and has particular strength in immuno-oncology. New drugs are vital, as roughly $6 billion of its annual revenue is at risk, as patents expire in the next few years and generic drugs start competing. Bristol-Myers Squibb stock yields 2.7% and at year-end it was Citadel's 14th-largest holding.

MGM Resorts International offers no dividend, but its stock has more than doubled over the past year, making it the hottest company in Las Vegas. (It does carry a lot of debt, though. That has been worrying some folks less lately, now that business is picking up in Las Vegas.) In its fourth quarter, MGM Resorts posted revenue up 10% over year-ago levels and $330 million in consolidated operating income, compared with a $425 million loss last year. MGM China is doing particularly well, with fourth-quarter revenue up 27%.

Kodiak Oil & Gas is a top operator in the promising Bakken Shale region, and has been boosting its production aggressively, thanks to fracking and other methods. In its fourth quarter, Kodiak's production doubled, with net income up 40% over year-ago levels (though still a penny shy of expectations). Its proven reserves have grown by 77% over the past year, too. Some worry, though, about Bakken oil being more flammable, the threat of increased regulation, and Kodiak's hefty debt load. Bulls, though, will point out that a stronger Kodiak is now able to fund growth from cash flow, not debt.