No Adverse Reproductive Effects Observed in Tree Swallows Exposed to Perfluoroalkyl Substances in Clarks Marsh, Michigan

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Perfluoroalkyl substance (PFAS) concentrations in tree swallows (Tachycineta bicolor) breeding at Clarks Marsh near a decommissioned U.S. Air Force base in Michigan were among the highest concentrations ever documented in birds indicating significant PFAS exposures.   In contrast to previous studies where reproductive impairment was documented at lower PFAS exposure, there were no adverse effects on reproductive success in this study.

To address this information gap, scientists from the U.S. Geological Survey (USGS) Ecologically-Driven Exposure Pathways Team quantified exposure, and determined if PFAS exposure was related to tree swallow reproductive success. Tree swallows were chosen as a model wild avian species because of their diet of aquatic invertebrates, which would lead to tissue concentrations directly reflecting bioavailable contaminants in sediments. In addition, tree swallows feed locally (within 0.5 kilometer of their nest boxes) and therefore reflect local rather than regional exposures to PFASs and other chemical stressors.

Installing nest boxes at Clarks Marsh, Michigan

Installing nest boxes at Clarks Marsh, Michigan.

(Credit: Christine M. Custer, USGS Upper Midwest Environmental Sciences Center. Public domain.)

The study site, Clarks Marsh, is on the south side of the former Wurtsmith Air Force Base in Oscoda Township, Michigan, and where PFAS occurrence has been previously documented in groundwater and nearby surface waters. The PFASs at this site originated from firefighting foams that were used during a 25-year period for training purposes.

Exposure to 13 PFAS congeners was measured in tree swallow tissues (plasma, liver, brain, egg, and diet) in each of 4 years at the study site. Biological endpoints, consistent with previous USGS studies in the Great Lakes watershed and Mississippi River basin, such as reproductive success, as percent hatching, were quantified. In addition, exposure biomarkers including ethoxyresorufin-O-dealkylase activity (EROD), genetic damage in red blood cells (measured as the amount of deoxyribonucleic acid [DNA] in each cell), and two thyroid hormone levels measured in plasma and the thyroid gland itself were measured.

Concentrations of PFASs in tree swallow tissues were greater at Clarks Marsh than at reference locations and at other locations throughout the US. The scientists noted that the tree swallows at the site had some of the highest PFAS concentrations reported in bird eggs except for the eggs of the great tit (Parus major) at a site near a PFAS manufacturing plant in Belgium.

Reproductive success was between 83 and 92 percent of the eggs hatching and not different than reproductive success at reference locations, and the daily failure rate (mortality) was less than the failure rate at 85 percent of 69 monitored sites across the Great lakes indicating a lack of reproductive effect. This lack of a reproductive effect is consistent with established toxicity reference values for PFASs. Similar to the reproductive outcomes, no differences were observed in EROD, DNA, or plasma and thyroid hormone levels in tree swallows between Clarks Marsh and reference locations indicating PFAS exposure at Clarks Marsh is not inducing EROD activity, genotoxicity, nor thyroid effects.

In contrast to this study, previous work reported a relation between PFAS exposure and reduced reproductive success for birds. For example, tree swallows had impaired reproduction based on a hatching success rate of 68 to 71 percent at sites in the Mississippi River basin where PFAS tissue concentrations were lower than those measured in Clark’s Marsh. The differences between studies may be because of exposures to other contaminants including other PFAS congeners that were not measured. In addition, there is a possibility that the combined effects of contaminant exposure and other stressors not assessed could have resulted in reduced reproduction at other sites.

This study, and the earlier studies by the Contaminant Biology and Toxic Substances Hydrology combined programs, in collaboration with the Great Lakes Restoration Initiative (GLRI), provide datasets and understanding of avian exposure to contaminants across the Great Lakes Region in a consistent and replicable manner. The Ecologically-Driven Exposure Pathways Team is continuing to study wildlife responses to multiple stressors including contaminants and pathogens in order to provide information critical to understanding the effects, if any, of exposure.

The USGS Contaminant Biology and Toxic Substances Hydrology combined programs funded this study, as well as the Great Lakes Restoration Initiative.