Western Ecological Research Center 

Publication Brief for Resource Managers

Contacts:

Dr. Josh Ackerman

Phone:

530-752-0485

Email:

jackerman@usgs.gov

Release:

March 2008

Davis Field Station, USGS Western Ecological Research Center, One Shields Avenue, University of California, Davis, CA 95616

Management Implications:

• These results show that fish-eating waterbirds are 

at risk to mercury contamination in San Francisco 

Bay and should be incorporated into risk-assessment strategies.

• Currently, management agencies are implementing large-scale plans to restore or enhance wetland 

habitats along San Francisco Bay’s margins. Restoration activities could increase the contamination of 

the aquatic biota within the estuary by accelerating 

microbial conversion of legacy inorganic mercury 

to methylmercury, the form which is highly toxic 

and most bioavailable to wildlife and humans. 

• Post-restoration monitoring of mercury concentrations in terns and effects of mercury on tern 

reproductive success is warranted to evaluate 

potential impacts of mercury on breeding terns and 

to determine alternative management strategies.

U.S. Department of the Interior

U.S. Geological Survey Publication Briefs online: http://www.werc.usgs.gov/pubbriefs/index.html

Mercury in Forster’s Terns in Relation to Space Use 

of San Francisco Bay Habitats

Fish-eating birds are good indicators of mercury contamination and risk to wildlife in aquatic food webs 

because they forage at a high trophic level and methylmercury biomagnifies through aquatic food chains. 

Despite a large body of literature assessing waterbird 

exposure to mercury, few studies have simultaneously 

examined mercury concentrations and space use in 

birds. To understand mercury exposure of waterbirds 

in San Francisco Bay, USGS scientists Dr. Josh Ackerman, Dr. Collin Eagles-Smith, Dr. John Takekawa, Jill 

Bluso, and USFWS scientist Terry Adelsbach investigated the mercury concentrations and habitat use of 

pre-breeding Forster’s terns using radio telemetry. They 

reported their results in a recent issue of Environmental 

Toxicology and Chemistry.

In 2005 and 2006, the authors collected blood and 

feathers from 122 Forster’s terns and radio-marked 

and tracked 72 terns to determine locations of dietary 

mercury uptake. Capture site and capture date were the 

most important factors explaining variation in blood 

mercury concentrations, followed by sex and year. Accordingly, radio telemetry data revealed that Forster’s 

terns generally remained near their site of capture and 

foraged in nearby salt ponds, managed and tidal marshes, and tidal flats. In contrast, capture site and capture 

date were not important factors explaining variation 

in feather mercury concentrations, probably because 

feathers were grown on their wintering grounds several 

months prior to our sampling. Instead, sex and year 

were the most important factors explaining mercury 

concentrations in breast feathers and sex was the most 

important factor for head feathers. Overall, 13% and 

22% of pre-breeding Forster’s terns were estimated to 

be at high-risk for deleterious effects due to mercury 

concentrations in blood and feathers, respectively. 

Breeding terns are likely to be even more at risk because blood mercury concentrations more than tripled 

during the 45-day pre-breeding time period. These data 

illustrate the importance of space use and tissue type in 

interpreting mercury concentrations in birds.

Ackerman, J.T., C.A. Eagles-Smith, J.Y. Takekawa, J.D. 

Bluso, and T.L. Adelsbach. 2008. Mercury concentrations in 

blood and feathers of pre-breeding Forster’s terns in relation 

to space use of San Francisco Bay habitats. Environmental 

Toxicology and Chemistry 27:897–908.