Hundreds of the stocks in the S&P 500 (SNPINDEX:^GSPC) pay a dividend, making the index a treasure trove for dividend investors searching for smart investing ideas. But when it comes to the highest-yielding stocks in the S&P, you'll find that all three of the top picks share a common trait: they're all in the same industry.

For a long time, you could find a diverse set of stocks topping the S&P 500's yield list. But with dividend cuts from Pitney Bowes and Cliffs Natural and the eviction of R.R. Donnelley from the index, the industry diversification among the best dividend payers has disappeared.

Rural telecoms lead the list
Left to top the dividend-paying list of stocks are well-known stocks from the rural telecom industry. Windstream (NASDAQ:WIN) is the highest-yielding of the bunch, with a yield of 12.4%. Alone among the three main rural telecom companies, Windstream has managed to avoid cutting its quarterly payout, having maintained its $0.25 per share quarterly dividend since December 2006. Even as earnings have badly lagged the amount of money it's returning to shareholders, Windstream has remained committed to sustaining its current dividend.

No. 2 yielding Frontier Communications (NASDAQ:FTR) hasn't been as lucky, having sustained two separate dividend cuts since 2010 that together have lopped 60% off its quarterly payout. Even the diminished dividend, however, is enough to produce a 9.2% yield for the stock. Frontier and Windstream both share many of the same growth challenges, with their dwindling traditional landline and basic telecom services businesses promising a decaying future unless the companies can replace their lost revenue with higher-margin services like broadband or enterprise-targeted telecom offerings. Like Windstream, Frontier hasn't managed to get its earnings up to match its dividend payout, raising concerns about its sustainability even at its lower level.

Taking third place is CenturyLink (NYSE:CTL), which yields just 6.5% after having cut its dividend by about 25% early this year. With the decrease, however, CenturyLink's dividend looks a lot more sustainable from a payout-ratio standpoint than its peers', albeit with the company still making larger payments than its current earnings per share. CenturyLink has also done a better job than its fellow rural telecoms at going beyond old-style telecom offerings, with its SAVVIS acquisition having opened the door to the fast-growing enterprise cloud-computing industry. If CenturyLink can keep moving in that direction, it could succeed where other telecoms will likely fail.

Finally, FirstEnergy (NYSE:FE) isn't a telecom company, but as an electric utility, its business has some of the same characteristics as its fellow dividend leaders on the S&P list. Its yield of 5.9% is among the best in the industry, but difficult conditions in the electricity-generation business have put a potential limit on its growth potential going forward. Unless conditions improve, shareholders might need to rely on their dividend income without great expectations of capital appreciation to go with it.

Don't count on high yields
High yields can be attractive, but they're often a precursor to painful dividend cuts. As recent turnover on the high-yield list shows, investors need to be careful before expecting that a dividend-yield leader will be able to maintain its payout in the future.

Fool contributor Dan Caplinger has no position in any stocks mentioned. You can follow him on Twitter: @DanCaplinger. The Motley Fool has no position in any of the stocks mentioned. Try any of our Foolish newsletter services free for 30 days. We Fools don't 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.