When birds die due to oil or chemical exposure at an oil company's storage or waste-water facility, the company may be prosecuted for violating the Migratory Bird Treaty Act. Exxon-Mobil, for example, recently pled guilty to killing 85 birds protected under the MTBA. The oil giant will pay $600,000 in fines, and several million more to implement a compliance plan to prevent bird deaths in the future.

Exxon-Mobil's not alone. Electric utilities are also prosecuted when protected birds are killed by poorly insulated transmission lines. And yet not all power produces are prosecuted for the accidental killing of protected birds.

As the Entergy Tribune's Robert Bryce detailed in the Wall Street Journal, wind power kills more protected birds than Exxon-Mobil's refineries, and yet gets a free pass.

A July 2008 study of the wind farm at Altamont Pass, Calif., estimated that its turbines kill an average of 80 golden eagles per year. The study, funded by the Alameda County Community Development Agency, also estimated that about 10,000 birds—nearly all protected by the migratory bird act—are being whacked every year at Altamont.

Altamont's turbines, located about 30 miles east of Oakland, Calif., kill more than 100 times as many birds as Exxon's tanks, and they do so every year. But the Altamont Pass wind farm does not face the same threat of prosecution, even though the bird kills at Altamont have been repeatedly documented by biologists since the mid-1990s.

The number of birds killed by wind turbines is highly variable. And biologists believe Altamont, which uses older turbine technology, may be the worst example. But that said, the carnage there likely represents only a fraction of the number of birds killed by windmills. Michael Fry of the American Bird Conservancy estimates that U.S. wind turbines kill between 75,000 and 275,000 birds per year. Yet the Justice Department is not bringing cases against wind companies.

The problem of bird kills from wind power are well documented. A 2001 report on avian mortality by the National Wind Coordinating Council estimated wind power was responsible for 33,000 bird kills per year, the vast majority of which are protected under federal law. The American Wind Energy Association estimates bird mortality rates are, on average, "one to six per year or less" per megawatt of wind power capacity in the United States. Given the U.S. had 25,000 megawatts of installed wind capacity in the U.S., wind power could be responsible for as many as 150,000 bird kills per year. How many will die if wind production increases ten-fold or more to meet proposed renewable energy mandates? (And will we consider that actual wind output can be far less than installed capacity.)

Wind power is hardly the only thing that kills birds. Bird kills are a problem with many tall structures, and other energy sources are hardly without their problems. All things considered, wind may be preferable to available alternatives (even if it cannot provide base load capacity) and could be an important part of America's energy supply in the future. Yet it seems clear that when it comes to killing protected birds, traditional energy companies face federal prosecution, while wind energy gets a pass.

One reason for the special treatment is that it is easier to reduce bird kills at traditional energy facilities than a wind farm. In Exxon-Mobil's case, netting can keep birds away from potential contamination sources. There's no comparably easy fix for wind farms -- at least not yet. So federal prosecutors may target enforcement efforts where they can maximize the environmental results. It's also possible that there's no political benefit to going after "green" energy.