We investigated mercury (Hg) blood concentrations in Bicknell’s thrush (Catharus bicknelli) and Swainson’s thrush (C. ustulatus), congeneric long-distance migratory songbirds, from 2000−2017 at a montane forest site in north-central Vermont. We analyzed variation in blood Hg of both species using mixed-effects models, incorporating atmospheric wet Hg deposition data from a nearby sampling location. Although Hg deposition varied among years and seasonally, we detected no temporal trend in either atmospheric deposition or blood Hg, nor evidence of a relationship between the two. Sampling date had the strongest effect on blood Hg concentration, which declined seasonally, followed by age and sex of the individual. The data did not support an effect of species. We believe that the absence of a clear relationship between local atmospheric deposition and thrush blood Hg concentrations suggests that Hg cycling dynamics, mechanisms of transfer, and timing of uptake by montane forest biota are complex and poorly understood. The blood Hg concentrations of ~0.07–0.1 μg/g we documented in Bicknell’s and Swainson’s thrush are below those found to negatively impact physiological or reproductive endpoints in other invertivorous terrestrial passerines. To better evaluate the validity of Bicknell’s thrush as a bioindicator of MeHg availability in montane forest ecosystems, we recommend (1) effects-based investigations, (2) a more robust understanding of Hg and MeHg cycling, (3) more clear geospatial and temporal links between Hg deposition and biotic uptake, and (4) more thorough documentation of Hg burdens across the species’ annual cycle.
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We thank our many outstanding field assistants who helped collect these data over the years, often under difficult conditions. The Stowe Mountain Resort provided invaluable logistical support for our field work. We are grateful for funding support received from the Charles E. & Edna T. Brundage Charitable Science and Wildlife Conservation Foundation, the Oakland Foundation, the Thomas Marshall Foundation, the USDA Forest Service Northeast Research Station, the U.S. Fish and Wildlife Service, the Vermont Monitoring Cooperative, and friends of the Vermont Center for Ecostudies. We also thank Kevin Regan of Biodiversity Research Institute for conducting lab analyses. Jim Duncan, Mike Finnegan and Miriam Pendleton provided helpful guidance in making available and interpreting atmospheric Hg deposition data from the Forest Ecosystem Monitoring Cooperative. Eric Miller and Jamie Shanley also provided helpful advice. We are grateful to Charles Driscoll, James Shanley, and two anonymous reviewers for constructive comments on earlier drafts of this manuscript.
Conflict of interest
The authors declare that they have no conflict of interest.
All applicable international, national, and/or institutional guidelines for the care and use of animals were followed. All procedures performed in the handling and blood sampling of thrushes in this study were in accordance with the ethical standards of the Vermont Center for Ecostudies, the U.S. Fish and Wildlife Service, and the University of Vermont.
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Rimmer, C.C., Lloyd, J.D., McFarland, K.P. et al. Patterns of blood mercury variation in two long-distance migratory thrushes on Mount Mansfield, Vermont. Ecotoxicology 29, 1174–1182 (2020). https://doi.org/10.1007/s10646-019-02104-3
- Mercury bioaccumulation
- Nearctic-Neotropical migratory songbirds
- Catharus bicknelli
- Montane forests