Do songbirds in wetlands show higher mercury bioaccumulation relative to conspecifics in non-wetland habitats?
Environmental conditions in wetlands facilitate favorable biogeochemical conditions for the conversion of inorganic mercury into methylmercury. For this reason, wetlands are increasingly classified as mercury hotspots, places where biota exhibit elevated mercury concentrations. While it is clear that wetlands play an important role in methylmercury production, factors such as geographic variation in mercury deposition, wetland type, and trophic dynamics can cause variation in mercury dynamics and bioaccumulation in biota occupying wetlands or connected to wetland trophic systems. Here, we use songbirds as bioindicators in a two-pronged approach aimed at evaluating the state of our understanding of mercury bioaccumulation by songbirds in wetland ecosystems. First, we use a case study in southeast Missouri to compare blood mercury concentrations in tree swallows (Tachycineta bicolor) and eastern bluebirds (Sialia sialis) occupying wetland and non-wetland habitats to test the hypothesis that songbirds in wetlands will have higher mercury bioaccumulation than those in non-wetlands. Adult tree swallows in wetlands had significantly higher blood mercury concentrations than those in non-wetlands; however, no difference between ecosystems was detected in eastern bluebirds. Second, we present a review of the current literature on mercury in songbirds in wetland ecosystems across North America. Mercury concentrations in songbirds varied among wetland types and with geographic location, often in an unpredictable manner. Mercury concentrations in songbird blood varied 3–10 fold at locations separated only by ~10 to several hundred kilometers. This magnitude of difference in blood mercury concentrations among wetlands exceeds documented differences between wetland and non-wetland ecosystems. Therefore, we caution against the automatic assumption that songbirds occupying wetlands will have higher mercury bioaccumulation than conspecifics living in other habitats.
KeywordsBioaccumulation Blood mercury Eastern bluebird Estuary Freshwater wetland Salt marsh
We would like to thank K. Cordell at Duck Creek Conservation Area for providing access to the site and workshop as well as employee assistance in setting up the nestbox trail. Kathy and Cliff Ham offered complete access to their farm property in Wayne County, MO (non-wetland site) and have been supportive of our research at Southeast Missouri State University (SEMO), for which we are incredibly grateful. A portion of our nestboxes were donated to R. Brasso by C. Fiedler, others were constructed for the project by D. Wood at SEMO. Field assistance was provided by SEMO graduate student K. Hixson and undergraduates L. Probst and E. Cordell. We would like to thank D. Cristol and D. Evers for inviting us to submit to this special issue.
Funding and support of laboratory and fieldwork awarded to R. Brasso from the Grants Research Funding Committee and College of Science, Technology, and Agriculture at Southeast Missouri State University. Additional funding provided by a Graduate Research Scholarship to K. Rittenhouse from the Audubon Society of Missouri (2017). Samples were collected in accordance with federal (#23903), state (Missouri Dept of Conservation, #16685, 17314), Missouri Dept of Natural Resources, and IACUC (Southeast Missouri State 16-001) permits awarded to R. Brasso.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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