Cooperative Fish and Wildlife Research Units Program: North Carolina
Education, Research and Technical Assistance for Managing Our Natural Resources

North Carolina Project

Assessing the relationship between acid precipitation, calcium depletion, and avian productivity in Great Smoky Mountains National Park

September 2004 - September 2010


Participating Agencies

  • USDA Forest Service

The primary objectives of this research are revised to: 1. Quantify the effects of acid deposition and subsequent thresholds of calcium depletion on the reproductive success of high elevation songbirds and the diversity and abundance of terrestrial snail populations in the southern Appalachians. 2. Determine the level of threat of mercury bioaccumulation in high elevation breeding songbirds and terrestrial snails in the southern Appalachians. Explanation: Research Objective #2 has been added because it has recently been determined that mercury (Hg) and methylmercury (MeHg) are important contaminants of wildlife populations. Inorganic mercury enters aquatic systems via atmospheric deposition where it is chemically converted to MeHg, a biologically active and toxic form. Recent research has shown that MeHg bioaccumulates through the aquatic foodweb from plankton, to invertebrates, to fish, and then to piscivorous vertebrates. Although recently there has been much improvement in understanding the mechanisms of mercury transfer and accumulation in freshwater aquatic systems, little is known about its role in the terrestrial system. Upland forest soils are natural sinks for atmospherically deposited mercury because the majority of mercury binds to organic and mineral soil particles. Mercury is concentrated primarily in the upper soil column. Research indicates that up to 60% of Hg that reaches lakes originates from the terrestrial watershed. Mercury loading is significantly higher in evergreen compared to deciduous habitats, and higher (2-5x) in mountainous areas in the northeastern United States compared to nearby low elevation areas. Consumption of contaminated food is the primary risk of mercury exposure to terrestrial vertebrates. Birds are at particularly high risk to mercury toxicity because many species are at high trophic levels, are long-lived, and are vulnerable to neurological and reproductive impacts from elevated Hg levels. The effects of mercury contamination in birds has been primarily studied in large migratory birds, wading birds, shorebirds, and raptors; few studies have addressed these issues in passerines, especially those species not associate with aquatic systems. In a multi-species analysis of mercury contamination, piscivorous birds were repeatedly shown to have some of the highest MeHg levels of the 38 species analyzed, however insectivorous birds in both aquatic and terrestrial habitats were also found with elevated levels. Although little research has focused on effects of mercury in passerines, in 2005 Rimmer et al. hypothesized that among terrestrial birds mercury contamination poses the highest risk to obligate insectivorous species. Rimmer found that blood mercury levels in Bicknell's Thrush (Catharus bicknelli) were correlated with Hg levels in leaf litter, demonstrating that dietary MeHg uptake occurs in passerines inhabiting terrestrial environments unassociated with standing water. Mercury accumulation in nestling Prothonotary Warblers (Protonotaria citrea) was also correlated with soil mercury concentrations around the nest. Rimmer further demonstrated that blood Hg levels in Bicknell's Thrush overlapped with those of Bald Eagles, indicating equivalent trophic status of a terrestrial-based insectivore with an aquatic-based piscivore. Rosten et al. found that resident insectivorous passerine species at contaminated sites in Europe accumulated higher mercury loads in eggs and nestlings than migratory or seasonal species. Although Bishop et al. claim that nestling birds are the best indicator of local contaminant trends, mercury levels in adult resident birds can also demonstrate chronic effects due to bioaccumulation. One limitation with the Rimmer et al. study is that they examined mercury levels in Neotropical migrants, and were thus unable to study chronic effects of mercury in resident adults. In birds, mercury loads are often sampled in specific organs, blood, or feathers, with mercury accumulation seen regardless of the material examined. Although adult females can reduce body burdens of mercury during egg production, depuration of MeHg most commonly occurs during feather growth, as MeHg binds strongly to keratin. Virtually all Hg in a feather is MeHg and reflects blood Hg levels at the time of molt. Because an acidified environment both enhances methylation of mercury and depletion of calcium, the adverse effects of acid precipitation may be complex. A recent study showed that Pied Flycatchers (Ficedula hypoleuca) were more sensitive to decreased levels of calcium-rich prey (i.e. snails) in a heavy-metal-polluted area. Thus, there is a pressing need to understand whether atmospheric pollution is contributing to both calcium limitation and mercury toxicity in southern Appalachian high elevation communities. The high elevations of Great Smoky Mountains National Park continue to report some of the highest rates of deposit of air-borne nitrate and sulfate in eastern North America. One effect of this well documented acidification is the loss of calcium, which is an essential element to most life. Observational and experimental studies in highly acidified sites in northern Europe have shown that losses of environment calcium are linked to reductions in populations of songbirds which depend on snail shell calcium for egg production. Similar research has not been conducted in the U.S., but it is necessary to assess the widespread effects of acid deposition on terrestrial animals. Researchers will use the Black-capped Chickadee as a model for evaluating the effects of acid precipitation on high elevation bird communities in the southern Appalachians. Black-capped Chickadee populations have been declining in the southern Appalachians over the past two decades, and they have disappeared as breeders from several high elevation sites where they once occurred. Objectives: (1) Assess whether Black-capped Chickadees nesting at high elevations in Great Smoky Mountains National Park are suffering from acid rain related calcium limitations during the breeding season. (2) Determine the sources of calcium for Black-capped Chickadees breeding at high elevations in the Park. (3) Determine if calcium limitation affects the behavior, habitat use, or breeding success of adult birds