Exposure and toxicity of environmental mercury to birds can be enhanced or lessened due to the available sources and forms of mercury and other species dependent factors such as life stage, migratory patterns, foraging and nesting behaviors, transfer of mercury from mothers to eggs, and sex. For example, mercury exposure can lead to sublethal toxicological effects that can influence parental nesting behaviors involved in egg incubation. However, the effects are species-specific and can be non-existent for some birds. These findings show how extensive the range of effects can be for a variety of different species and help to differentiate the actual vs the perceived effects of contaminants on individuals, populations, and entire ecosystems.
Methylmercury exposure poses a perceived health risk to humans and wildlife globally. As a neurotoxin, methylmercury has been linked to sublethal adverse health outcomes in humans and other animals. However, exposure to methylmercury alone does not determine the health risk to humans or wildlife—numerous pathways and processes in the environment and within an organism can alter its toxicity. This complexity and information gap can make it difficult to understand or predict where methylmercury exposure poses the greatest health risks.
The U.S. Geological Survey’s (USGS) Ecologically-Driven Exposure Pathways Team, with partners from academia, industry, and tribal, State, and Federal agencies, provides tools to facilitate understanding of mercury and other toxicant exposure and effects on birds and other wildlife. The team has studied contaminant exposure pathways for numerous wildlife species, but in this article, the focus is on environmental mercury exposure in wild birds. Some of the team’s most recent contributions provide information on when and where methylmercury exposure occurs, pathways of contaminant bioaccumulation, and whether the exposure affects bird health.
For example, the team examined the drivers of methylmercury exposure in 52 songbird species to understand factors affecting exposure. Mercury accumulation in blood and feathers varied among songbird species, illustrating the importance of species-specific ecology as a predominant driver of mercury bioaccumulation among all animals, including birds. Mercury accumulation in blood and feathers also was dependent on diet and when or where the songbirds reside and feed. For example, blood mercury concentrations were greater for those birds feeding at higher trophic levels owing to the propensity of mercury to biomagnify through food chains.
Beyond the environmental pathways and drivers of mercury exposure, how a bird takes in, processes, and excretes contaminants influences exposure and potential effects. The team measured maternal transfer of methylmercury to bird embryos and not all species pass the same amount of mercury to their eggs, even if there are similar contaminant levels in the mothers. For example, the proportion of mercury transferred from tree swallow (Tachycineta bicolor) mothers to their eggs was 17–107 percent greater than that transferred by house wren (Troglodytes aedon) mothers. In these cases, the risk of toxicity for the adult female and the embryo changes simply because of the amount of the contaminant passed to offspring. Moreover, the team has reported that other physiological processes, such as feather molting, affect how long the mercury remains within the birds. For example, blood-mercury concentrations in Western and Clark’s grebes (Aechmophorus occidentalis) decreased by 60 percent from spring to fall, consistent with the onset of feather molt.
Mercury effects on birds often are sublethal and can include changes in behaviors. For example, the team determined that parental nesting behaviors involved in egg incubation, which plays a significant role in embryonic development, were altered in a population of tree swallows highly exposed to methylmercury but not in Forster’s terns (Sterna forsteri). Less time spent incubating can lower egg temperatures, slow embryonic development, and potentially lengthen the incubation period, which may negatively influence reproduction.
These interconnected studies are part of a goal of the USGS Contaminant Biology and Toxic Substances Hydrology combined programs to provide the science needed to understand the actual, as opposed to the perceived, risks that toxicant exposures pose to the health of fish, wildlife, livestock, pets, and humans. The Ecologically Driven Environmental Pathways Team applies their expertise in avian ecology, reproductive ecology, food web ecology, aquatic ecology, hydrology, toxicology, and environmental chemistry to answer complex questions related to how, when, and where contaminant exposure occurs. The tools, data, and understanding they provide helps to increase consistency, comparability, and reduce variability—all necessary to identify the actual effects of contaminants on individuals, populations, and entire ecosystems that could be used to predict where exposure poses the greatest health risks.
These studies were funded by the USGS Contaminant Biology and Toxic Substances Hydrology combined programs and several other funding sources listed in each reference.
Below are other science projects associated with this project.
Ecologically-Driven Exposure Pathways Science Team
Sublethal Effects of Contaminants in Aquatic Food Webs—Research Challenges and Considerations for Future Studies
Roadmap to Understanding Factors Influencing Mercury Exposure and Adverse Health Effects
Long-term Studies Examine Contaminant Exposure and Reproduction of Ospreys Nesting in Two Large United States Estuaries
New Method Can Measure Naturally Occurring Element Exposure in Hummingbirds Without Harm
Body Symmetry in Forster's Terns Related to Mercury Exposure
Efficacy of Eggshell Analyses as a Nonlethal Method to Estimate Mercury Exposure in Bird Embryos
Low Levels of Contaminants Found in Great Lakes Tree Swallow Nestlings
Synthesis of Mercury Distribution and Bioaccumulation Across Western North America
- Overview
Exposure and toxicity of environmental mercury to birds can be enhanced or lessened due to the available sources and forms of mercury and other species dependent factors such as life stage, migratory patterns, foraging and nesting behaviors, transfer of mercury from mothers to eggs, and sex. For example, mercury exposure can lead to sublethal toxicological effects that can influence parental nesting behaviors involved in egg incubation. However, the effects are species-specific and can be non-existent for some birds. These findings show how extensive the range of effects can be for a variety of different species and help to differentiate the actual vs the perceived effects of contaminants on individuals, populations, and entire ecosystems.
Sources/Usage: Public Domain.Yellow warbler sampled for methylmercury concentrations in blood and feathers and then released without harm. Methylmercury exposure poses a perceived health risk to humans and wildlife globally. As a neurotoxin, methylmercury has been linked to sublethal adverse health outcomes in humans and other animals. However, exposure to methylmercury alone does not determine the health risk to humans or wildlife—numerous pathways and processes in the environment and within an organism can alter its toxicity. This complexity and information gap can make it difficult to understand or predict where methylmercury exposure poses the greatest health risks.
The U.S. Geological Survey’s (USGS) Ecologically-Driven Exposure Pathways Team, with partners from academia, industry, and tribal, State, and Federal agencies, provides tools to facilitate understanding of mercury and other toxicant exposure and effects on birds and other wildlife. The team has studied contaminant exposure pathways for numerous wildlife species, but in this article, the focus is on environmental mercury exposure in wild birds. Some of the team’s most recent contributions provide information on when and where methylmercury exposure occurs, pathways of contaminant bioaccumulation, and whether the exposure affects bird health.
For example, the team examined the drivers of methylmercury exposure in 52 songbird species to understand factors affecting exposure. Mercury accumulation in blood and feathers varied among songbird species, illustrating the importance of species-specific ecology as a predominant driver of mercury bioaccumulation among all animals, including birds. Mercury accumulation in blood and feathers also was dependent on diet and when or where the songbirds reside and feed. For example, blood mercury concentrations were greater for those birds feeding at higher trophic levels owing to the propensity of mercury to biomagnify through food chains.
Sources/Usage: Public Domain.Photograph showing tree swallow nest with temperature data logger used to measure parental incubation periods. Beyond the environmental pathways and drivers of mercury exposure, how a bird takes in, processes, and excretes contaminants influences exposure and potential effects. The team measured maternal transfer of methylmercury to bird embryos and not all species pass the same amount of mercury to their eggs, even if there are similar contaminant levels in the mothers. For example, the proportion of mercury transferred from tree swallow (Tachycineta bicolor) mothers to their eggs was 17–107 percent greater than that transferred by house wren (Troglodytes aedon) mothers. In these cases, the risk of toxicity for the adult female and the embryo changes simply because of the amount of the contaminant passed to offspring. Moreover, the team has reported that other physiological processes, such as feather molting, affect how long the mercury remains within the birds. For example, blood-mercury concentrations in Western and Clark’s grebes (Aechmophorus occidentalis) decreased by 60 percent from spring to fall, consistent with the onset of feather molt.
Mercury effects on birds often are sublethal and can include changes in behaviors. For example, the team determined that parental nesting behaviors involved in egg incubation, which plays a significant role in embryonic development, were altered in a population of tree swallows highly exposed to methylmercury but not in Forster’s terns (Sterna forsteri). Less time spent incubating can lower egg temperatures, slow embryonic development, and potentially lengthen the incubation period, which may negatively influence reproduction.
These interconnected studies are part of a goal of the USGS Contaminant Biology and Toxic Substances Hydrology combined programs to provide the science needed to understand the actual, as opposed to the perceived, risks that toxicant exposures pose to the health of fish, wildlife, livestock, pets, and humans. The Ecologically Driven Environmental Pathways Team applies their expertise in avian ecology, reproductive ecology, food web ecology, aquatic ecology, hydrology, toxicology, and environmental chemistry to answer complex questions related to how, when, and where contaminant exposure occurs. The tools, data, and understanding they provide helps to increase consistency, comparability, and reduce variability—all necessary to identify the actual effects of contaminants on individuals, populations, and entire ecosystems that could be used to predict where exposure poses the greatest health risks.
These studies were funded by the USGS Contaminant Biology and Toxic Substances Hydrology combined programs and several other funding sources listed in each reference.
Sources/Usage: Public Domain.Clark's grebe sitting on a nest at Thermalito Afterbay, California. - Science
Below are other science projects associated with this project.
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...Sublethal Effects of Contaminants in Aquatic Food Webs—Research Challenges and Considerations for Future Studies
U.S. Geological Survey (USGS) and academic scientists partnered to identify challenges and provide considerations for future scientific study designs to advance our understanding of the often subtle sublethal effects of contaminants on individuals, populations, communities, and entire aquatic food webs.Roadmap to Understanding Factors Influencing Mercury Exposure and Adverse Health Effects
In a comprehensive overview, scientists explain that human and wildlife exposure and toxicological responses to mercury are dependent on factors that operate across global, individual, and molecular scales. They provide a roadmap for unified research to facilitate a better understanding of human and wildlife health risks from mercury exposure.Long-term Studies Examine Contaminant Exposure and Reproduction of Ospreys Nesting in Two Large United States Estuaries
In a series of studies from 2010 to 2018, U.S. Geological Survey (USGS) scientists detected low levels of legacy contaminants and pharmaceuticals in osprey (Pandion haliaetus) and their food chain within the Chesapeake and Delaware River estuaries. Osprey reproductive success increased during the same period and was determined to be adequate to sustain a stable population in both estuaries.New Method Can Measure Naturally Occurring Element Exposure in Hummingbirds Without Harm
Seventeen naturally occurring trace elements, including those associated with adverse health impacts when birds are exposed to toxic levels (iron, lead, mercury, selenium, zinc, cadmium, and arsenic) were measured in small birds without harm. The nonlethal method was developed for hummingbirds, with Anna's hummingbirds (Calypte anna) as a test species and can be more broadly applied to other...Body Symmetry in Forster's Terns Related to Mercury Exposure
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.Efficacy of Eggshell Analyses as a Nonlethal Method to Estimate Mercury Exposure in Bird Embryos
U.S. Geological Survey (USGS) scientists evaluated a nonlethal method to estimate mercury in the embryos of 23 bird species using mercury content in eggshells. This method was effective for a wide range of ages and species.Low Levels of Contaminants Found in Great Lakes Tree Swallow Nestlings
Tree swallow nestlings at most study sites in the Great Lakes basin were minimally exposed to organic contaminants.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). - Publications