Feathers of Peregrine Falcons in Nevada reveal mercury contamination

Researchers monitoring a recovering population of Peregrine Falcons in southern Nevada have discovered that the birds are contaminated with mercury, an element that has been correlated with increased mortality or reduced reproductive success in raptors and wading birds.

The species was federally listed as Endangered from 1973 to 1999 and had been considered extirpated in Nevada from the 1950s until 1985. Then a pair was discovered breeding in Lake Mead National Recreation Area, east of Las Vegas. The population has been rebounding steadily ever since.

Joseph G. Barnes and Shawn L. Gerstenberger of the University of Nevada, Las Vegas, assessed concentrations of total mercury not only in the feathers of adult and young falcons but also in the feather remains of birds eaten by the falcons. Then the researchers compared levels from territories near Lakes Mead and Mohave with those from the surrounding desert.

All of the Peregrines were contaminated, the researchers found, but falcons whose eyries were close to the lakes carried higher loads than those nesting farther away, and the more aquatic birds the Peregrines took as prey, the greater their concentration was.

Eared Grebes appear to be the single largest contributor of mercury. The birds are generally present in large numbers on Lakes Mead and Mohave during April and May, they make up almost 15 percent of the falcons’ diet, and they had the highest mean mercury concentration of 94 prey species analyzed.

What’s more, nearly half of the North American population stages between August and January on the Great Salt Lake, in Utah. Recent studies have revealed that the lake’s surface water contains the highest methyl mercury concentrations in the country, and levels of mercury in the livers of waterfowl species that overwinter on the lake were among the highest ever published for those species.

A version of this article appeared in the June 2015 issue of BirdWatching. Subscribe.

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Originally Published April 9, 2015