Vestas Wind Systems and Siemens Gamesa are giants of the wind-power industry, building mammoth turbines that rise high into the air and power more and more homes. But disappointing earnings reports from the two companies this week indicated that even they are struggling to adapt to a fast-changing sector.

Wind power is an increasingly important source of electricity around the world, and prices for the technology are dropping fast. But belt-tightening governments across Europe and North America are phasing out subsidies and tax incentives that had helped the industry grow, squeezing companies like Vestas and Siemens Gamesa in the process.

On Thursday, Vestas, the world’s largest maker of wind turbines, said its revenue in the third quarter fell 6 percent compared with the same period a year ago, to 2.7 billion euros, or $3.1 billion. Profit dropped 18 percent, to about €250 million, Vestas said. The figures sent the Danish company’s shares plummeting by as much as 20 percent.

The Vestas results came just days after Siemens Gamesa Renewable Energy — the recently formed company combining the wind-power units of Siemens, the German conglomerate, and Gamesa of Spain — reported a €147 million loss for the third quarter. The Madrid-listed company also said it would have to shed 6,000 jobs.