Body symmetry of Forster's terns (Sterna forsteri) in San Francisco Bay was related to blood and feather mercury concentrations. Body asymmetry can affect a bird's fitness by reducing flight efficiency, thus increasing energetic costs (especially during migration) and interrupting normal feeding and breeding behaviors.
Mercury is a widespread global contaminant, and when converted to its organic form (methylmercury) it biomagnifies through food webs. As a result, top predators and species that forage in habitats favorable to methylmercury production, such as estuaries, are at particular risk to mercury exposure. The toxicological effects of exposure manifest in numerous endpoints that can influence a bird's survival or reproduction. One effect, variation in birds' body symmetry – also known as fluctuating asymmetry, can be an indicator of developmental problems in response to a variety of stressors, including contaminants in the environment.
Scientists determined whether mercury exposure in four waterbird species from San Francisco Bay, California, was related with indices of fluctuating asymmetry. San Francisco Bay has elevated concentrations of mercury in water and sediment, as well as fish and estuarine birds. U.S. Geological Survey (USGS) researchers measured five paired morphometric features, such as wing length and primary feather length) and determined the relation between the morphometric features and mercury concentrations in whole blood or breast feathers.
Forster's terns (which have the highest mercury concentrations in the Bay) had the highest asymmetry index (the most asymmetric) across the four species tested. Fluctuating asymmetry increased with total mercury concentrations in whole blood and breast feathers of Forster's terns, but there was no relationship between measures of fluctuating asymmetry and total mercury concentrations in the three other species (American avocets (Recurvirostra americana), black-necked stilts (Himantopus mexicanus), and Caspian terns (Hydroprogne caspia).
The results of this study indicate that mercury contamination may act as an environmental stressor during development and feather growth, contributing to fluctuating asymmetry of some species of highly contaminated waterbirds, particularly Forster's terns (Sterna forsteri).
There are unanswered questions about the mechanism by which mercury influences fluctuating asymmetry, as well as how asymmetry influences the survival of birds. These unanswered questions are potential next steps for understanding the actual adverse health effects of mercury exposure on water birds. Fluctuating asymmetry due to mercury exposure may add an additional environmental stressor during the birds' development, potentially affecting their ability to fly efficiently and thus influencing normal feeding, and reproductive behaviors.
This study was funded by the USGS Contaminant Biology Program and Wildlife: Terrestrial and Endangered Resources Programs, and CALFED Ecosystem Restoration Program.
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North American and European Atmospheric Mercury Declines Explained by Local and Regional Emission Reductions
Comprehensive Assessment of Mercury in Streams Explains Major Sources, Cycling, and Effects
Mercury in Fish from 21 National Parks in the West
Complex Response to Decline in Atmospheric Deposition of Mercury
- Overview
Body symmetry of Forster's terns (Sterna forsteri) in San Francisco Bay was related to blood and feather mercury concentrations. Body asymmetry can affect a bird's fitness by reducing flight efficiency, thus increasing energetic costs (especially during migration) and interrupting normal feeding and breeding behaviors.
Mercury is a widespread global contaminant, and when converted to its organic form (methylmercury) it biomagnifies through food webs. As a result, top predators and species that forage in habitats favorable to methylmercury production, such as estuaries, are at particular risk to mercury exposure. The toxicological effects of exposure manifest in numerous endpoints that can influence a bird's survival or reproduction. One effect, variation in birds' body symmetry – also known as fluctuating asymmetry, can be an indicator of developmental problems in response to a variety of stressors, including contaminants in the environment.
Sources/Usage: Public Domain.Photo of Foster's Tern (Sterna forsteri) while hunting. Forster’s terns are among the most at-risk wildlife species to mercury toxicity in western North America. Photo Credit: Josh T. Ackerman, USGS, Western Ecological Research Center. Scientists determined whether mercury exposure in four waterbird species from San Francisco Bay, California, was related with indices of fluctuating asymmetry. San Francisco Bay has elevated concentrations of mercury in water and sediment, as well as fish and estuarine birds. U.S. Geological Survey (USGS) researchers measured five paired morphometric features, such as wing length and primary feather length) and determined the relation between the morphometric features and mercury concentrations in whole blood or breast feathers.
Forster's terns (which have the highest mercury concentrations in the Bay) had the highest asymmetry index (the most asymmetric) across the four species tested. Fluctuating asymmetry increased with total mercury concentrations in whole blood and breast feathers of Forster's terns, but there was no relationship between measures of fluctuating asymmetry and total mercury concentrations in the three other species (American avocets (Recurvirostra americana), black-necked stilts (Himantopus mexicanus), and Caspian terns (Hydroprogne caspia).
The results of this study indicate that mercury contamination may act as an environmental stressor during development and feather growth, contributing to fluctuating asymmetry of some species of highly contaminated waterbirds, particularly Forster's terns (Sterna forsteri).
There are unanswered questions about the mechanism by which mercury influences fluctuating asymmetry, as well as how asymmetry influences the survival of birds. These unanswered questions are potential next steps for understanding the actual adverse health effects of mercury exposure on water birds. Fluctuating asymmetry due to mercury exposure may add an additional environmental stressor during the birds' development, potentially affecting their ability to fly efficiently and thus influencing normal feeding, and reproductive behaviors.
This study was funded by the USGS Contaminant Biology Program and Wildlife: Terrestrial and Endangered Resources Programs, and CALFED Ecosystem Restoration Program.
- Science
Related science listed below.
Ecologically-Driven Exposure Pathways Science Team
The Ecologically-Driven Exposure pathways team identifies internal and external pathways and processes that can alter exposure and toxicity of contaminants and pathogens to wildlife the environment. That information is used to distinguish actual health risks to fish and wildlife. If actual risks are identified this project will inform how to economically and effectively minimize risk by providing...Synthesis of Mercury Distribution and Bioaccumulation Across Western North America
The unique combination of natural processes and human activities present in western North America can strongly affect the exposure of fish and birds to methylmercury, the most toxic form of mercury (Hg).North American and European Atmospheric Mercury Declines Explained by Local and Regional Emission Reductions
Recent findings from a consortium of university, State, and U.S. Geological Survey (USGS) scientists indicate that declining atmospheric concentrations of mercury (Hg) can be explained by the phaseout of mercury in many commercial products and by reduced emissions from utilities over the past two decades.Comprehensive Assessment of Mercury in Streams Explains Major Sources, Cycling, and Effects
A new USGS report, Mercury in the Nation's Streams—Levels, Trends, and Implications, presents a comprehensive assessment of mercury contamination in streams across the United States. It highlights the importance of environmental processes, monitoring, and control strategies for understanding and reducing stream mercury levels. This report summarizes selected stream studies conducted by the U.S...Mercury in Fish from 21 National Parks in the West
U.S. Geological Survey (USGS) and National Park Service (NPS) scientists collaborated in the first study to measure mercury in fish from remote places in 21 National Parks spanning 10 Western States, including Alaska. Mercury levels in fish generally were low, but were elevated in some local areas, including two parks in Utah and Alaska where samples taken from sport fish exceeded the U.S...Complex Response to Decline in Atmospheric Deposition of Mercury
U.S. Geological Survey (USGS) scientists found that mercury concentrations in shallow waters and methylmercury (MeHg) concentrations in fish in four lakes in Voyageurs National Park, Minnesota, were not consistent with decreases in the wet atmospheric deposition of mercury recorded at nearby monitoring stations for over a decade. Methylmercury is a toxic form of mercury (Hg) that accumulates and... - Data
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