When Are Bird Chicks at Highest Risk of Mercury Toxicity?

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Laboratory studies have shown that methylmercury exposure can impair bird chick behavior, health, growth and survival. For researchers studying the health of San Francisco Bay wetlands -- some of which still contains mercury contamination resulting from historic mining activities -- it is important to learn how wild bird chicks experience high mercury concentrations in their body.

Now, a study by biologists at the USGS Western Ecological Research Center has found that for three San Francisco Bay waterbird species, mercury toxicological risk is highest when chicks hatch and after they fledge feathers.

The study was conducted at the Don Edwards San Francisco Bay National Wildlife Refuge and Eden Landing Ecological Reserve in south San Francisco Bay. Both refuges are managed as part of the South Bay Salt Pond Restoration Project -- the largest tidal wetland restoration project on the West Coast, with many USGS research projects informing its management plans.

Biologists Josh Ackerman and Mark Herzog of the WERC Davis Field Station conducted the study with their colleague Collin Eagles-Smith of the USGS Forest and Rangeland Ecosystems Science Center.

Together, they examined total mercury and methyl-mercury concentrations in blood, liver, kidney, muscle, and feathers of 111 Forster’s tern (Sterna forsteri), 69 black-necked stilt (Himantopus mexicanus) and 43 American avocet (Recurvirostra americana) chicks as they aged from hatching through postfledging at wetlands that had either low or high mercury contamination in San Francisco Bay. 

Forster's tern fledglings

Forster's tern (Sterna forsteri) fledglings. (Credit: Josh Ackerman, USGS Western Ecological Research Center. Public domain.)

They found that mercury concentrations in internal tissues were highest immediately after hatching, which would be due to mercury in eggs deposited by the mother bird's body. This mercury concentration then rapidly declines as chicks aged. As chicks grew in size and gained body mass, the additional tissue actually diluted their mercury concentrations, and mercury also was transfered into growing feathers.

However, mercury concentrations increased again as chicks fledged and feather growth slowed. Chicks continued to accumulate mercury through their diet, and the rate of mercury excreted into feathers slowed with decreased feather growth.

This “U-shaped” pattern of mercury concentrations from hatching to fledging indicates that juvenile birds may be at highest risk to methylmercury toxicity shortly after hatching and again after fledging.

Granted, this study looked only at three out of the many bird species that breed and feed in south San Francisco Bay, but it's another important step towards understanding how contaminants interact with our wildlife and national refuge lands.

-- Ben Young Landis

American avocets

American avocets (Recurvirostra americana) in south San Francisco Bay. (Credit: Paul Laustsen, USGS. Public domain.)