Wind energy is one of the cleanest and increasingly cost-effective ways to generate electricity.
As countries seek to reduce carbon emissions, more and more wind turbines are being built.
But wildlife biologists in the US say the devices are killing large numbers of birds, including many species protected by law.
Now US companies are working on new wind turbine models, aiming to make them less dangerous for wildlife.
Al Jazeera’s Tom Ackerman reports from northern California.