Barnes’ latest findings were published in February in the Journal of Raptor Research.

More research is needed to determine how much mercury peregrine falcons can tolerate, Barnes said. But studies of other raptors suggest concentrations of between 5 and 15 parts per million can significantly reduce breeding.

Barnes documented average mercury levels of 17 ppm in adult peregrines at Lake Mead near Las Vegas, and 23 ppm in adult peregrines along the Washington coast. Even Falcons migrating south from remote Greenland, Alaska and northern Canada, far from industrial pollution sources, carried about 10 ppm of mercury in their feathers on average, he said.

Peregrines are “one of the most impacted species when it comes to contamination,” Barnes said because they feed on other birds that have been exposed to mercury.

At Lake Mead, favourite peregrine prey is the eared grebe, a small water bird that migrates through the area by the hundreds of thousands.

Barnes said most grebes at Lake Mead also spend time in mercury-laden waters of Utah’s Great Salt Lake.

The peregrine falcon hunts almost anything that flies, including birds and bats. It dives on prey from above with wings closed, enabling it to reach speeds of more than 200 mph (322 kph).

Barnes called peregrines an indicator of overall ecosystem health because they are found on every continent except Antarctica and they feed on a wide variety of species, from bats to hummingbirds to Canada geese.

Ryan Bourbour, a biologist and doctoral student at the University of California, Davis, has also studied mercury exposure in North American raptors. He said even low-level exposure can affect human and wildlife health.

Assessing exposure levels in top predators, such as peregrine falcons, “may have conservation implications and even human health implications,” he said.

Most human mercury exposure in the US is by eating fish containing methylmercury, which forms when bacteria react with mercury in water, soil, or plants. Elemental and methylmercury are toxic to the nervous system, and inhaling mercury vapour can cause a variety of symptoms or even death.

So far, the falcons Barnes is studying show no ill effects from exposure.

He said he has identified well over 90 different prey species from feathers collected from falcon nesting sites around lakes Mead and Mohave.

Though mercury occurs naturally, Barnes said research suggests levels have increased by 300 per cent since the start of the industrial revolution in the late 1700s.

“Peregrines are sort of the canary in the coal mine here,” he said.