Science to Help Understand Exposure and Toxicological Effects of Environmental Mercury to Representative Birds

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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.

Scientist holding a yellow warbler

Yellow warbler sampled for methylmercury concentrations in blood and feathers and then released without harm.

(Credit: Sarah R. Lemelin, U.S. Geological Survey. Public domain.)

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.

tree swallow nest with temperature data logger

Photograph showing tree swallow nest with temperature data logger used to measure parental incubation periods.

(Credit: Christopher "Alex" Hartman, U.S. Geological Survey, Western Ecological Research Center. Public domain.)

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.

Clark's Grebe at Thermalito Afterbay, CA

Clark's grebe sitting on a nest at Thermalito Afterbay, California.

(Credit: Alex Hartman, U.S. Geological Survey. Public domain.)