One of the most commonly repeated criticisms of wind power is that it kills birds. The giant spinning turbines are basically bird death traps—and often they cut through prime flying space, making the carnage even worse. At least that's the story. But how many birds really do die?

If you look around for statistics about bird deaths from wind turbines get you wildly different numbers. Some say just 10,000 birds a year meet their end at the hands (blades) of the wind industry. Others ramp that number up to 600,000. Now, a new study tried to actually use science to estimate.

Of course, they didn't go to each turbine and count how many little feathered bodies they found at the base. Instead, they combed the literature for all the studies they could find on bird deaths, and tried to combine them into an estimate. This meant searching for fun things like “'bird AND wind turbine' with 'collision,' 'mortality,'fatality,' 'carcass,' and 'post-construction.'" And then—even more cheerful—searching all those terms again, but "with 'bird' replaced by 'avian' and 'wildlife'; and 'turbine' replaced by 'farm,' 'facility' and 'energy.'"

In the end, using 58 mortality estimates that met their criteria, they came up with an estimate. According to the current literature somewhere between 140,000 and 328,000 birds die each year from collisions with wind turbines. That's not all, explains the blog Natural Reactions:

In addition, it appears that there is a greater risk of fatal collisions with taller turbines. This is a real problem, as larger wind turbines may provide more efficient energy generation. Consequently, it is expected that new wind farms will contain even bigger turbines, which will result in even more bird deaths. Future developments therefore will have to give very careful consideration to potential wildlife impacts when planning the type of turbine to install.

The estimate, and conclusions, don't let wind turbines off the hook. And with recent rulings to try and protect certain species from the spinning blades, the scrutiny will probably continue when it comes to bird deaths due to wind power. But at least now there's a scientifically derived number for those deaths.

More from Smithsonian.com:

Do Wind Turbines Need a Rethink?

Scientists Save Bats and Birds from Wind Turbine Slaughter