Mercury contamination is a widespread environmental issue, and the accumulation of methylmercury poses significant risks to wildlife, particularly birds. Birds are often used as bioindicators of environmental mercury contamination and during ecological risk assessments. In a recent study published in Environmental Toxicology and Chemistry, USGS researchers have derived new toxicity reference values and introduced “The Bird Mercury Tool,” a novel resource to assess the effects of mercury on birds.

Comprehensive Study and Meta-Analysis

USGS researchers conducted a comprehensive review of 168 studies, summarizing data on the effects of methylmercury on birds. They performed a series of Bayesian hierarchical meta-analyses to develop new toxicity reference values for multiple bird tissue residues and dietary mercury levels. The study quantified the lethal and sublethal effects of methylmercury on birds across nine biologically relevant endpoint categories and three age classes (eggs, juveniles, and adults).The researchers found that effective mercury concentrations that caused a 10% reduction in the production of juvenile offspring, histology endpoints, and biochemical markers were substantially lower than those affecting survival and behavior of juveniles and adults. Within the egg age class, survival was the most sensitive endpoint. Body morphology was not particularly sensitive to mercury.
 

New Toxicity Reference Values

Toxicity reference values were developed using a category that combined survival and reproduction endpoints for juveniles because they were more sensitive to mercury toxicity than eggs or adults. The study established the following mercury concentrations (μg/g) that cause varying levels of injury to birds:

• Low injury (1% reduction): 0.09 in adult blood, 0.04 in eggs, 0.16 in maternal diet
• Moderate injury (5% reduction): 0.6 in adult blood, 0.3 in eggs, 0.6 in maternal diet
• High injury (10% reduction): 1.3 in adult blood, 0.7 in eggs, 1.1 in maternal diet
• Severe injury (20% reduction): 3.2 in adult blood, 1.8 in eggs, 2.4 in maternal diet

The Bird Mercury Tool

Using the dose-response relationships and new toxicity reference values, USGS researchers developed “The Bird Mercury Tool.” This tool allows practitioners to quantify the injury to birds caused by mercury pollution by entering mercury concentrations in bird tissues or diet, along with associated metadata. The tool then calculates the estimated injury to birds based on the derived statistical models, providing a simplified way to assess the effect of environmental mercury contamination on bird populations.

Management Implications

• New toxicity reference values for the effects of methylmercury on birds were developed.
• These results can be used to quantify injury
  to birds caused by mercury pollution.
• “The Bird Mercury Tool” can be used to both quantify injury to birds and estimate the percentage of the sampled bird population that might be considered injured due to mercury contamination, such as during Natural Resource Damage Assessment (NRDA) cases.