Multiple Lines of Evidence Risk Assessment of Terrestrial Passerines Exposed to PCDFs and PCDDs in the Tittabawassee River Floodplain, Midland, Michigan, USA

A site-specific multiple lines of evidence risk assessment was conducted for house wrens (Troglodytes aedon) and eastern bluebirds (Sialia sialis) along the Tittabawassee River downstream of Midland, Michigan, where concentrations of polychlorinated dibenzofurans (PCDFs) and polychlorinated dibenzo-p-dioxins (PCDDs) in flood-plain soils and sediments are greater compared to upstream areas and some of the greatest anywhere in the world. Lines of evidence supporting the population-level assessment endpoints included site-specific dietary- and tissue-based exposure assessments and population productivity measurements during breeding seasons 2005-2007. While a hazard assessment based on site-specific diets suggested that populations residing in the downstream floodplain had the potential to be affected, concentrations in eggs compared to appropriate toxicity reference values (TRVs) did not predict a potential for population-level effects. There were no significant effects on reproductive success of either species. The most probable cause of the apparent difference between the dietary- and tissue-based exposure assessments was that the dietary-based TRVs were overly conservative based on intraperitoneal injections in the ring-necked pheasant. Agreement between the risk assessment based on concentrations of PCDFs and PCDDs in eggs and reproductive performance in both species supports the conclusion of a small potential for population-level effects at this site.