You are what you eat and where you live: how territory size and diet relate to mercury concentration in riparian songbirds

Abstract

Mercury (Hg) is a pollutant that has become a large threat to many ecosystems and the organisms that live within them. Mercury is a known neurotoxin that can cause extensive central nervous system damage, which can impact the behavior and survival of many species. Inorganic mercury is often deposited into the atmosphere by anthropocentric sources and can easily enter water sources through precipitation. These inorganic forms of mercury are then methylated by bacteria in anoxic environments, creating an organic form of mercury called methylmercury (MeHg). Methylmercury accumulates in individuals and easily moves up through the trophic levels. As a result, the bioavailability of methylmercury in anoxic environments becomes a pressing issue within aquatic food chains. Riparian songbirds, due to their mixed diet and the fact that they occupy both aquatic and terrestrial habitats, are often used to monitor methylmercury availability within these environments, but very little is known about how territory location can influence Hg exposure. In June and July 2018, we studied riparian songbirds at two sites in Acadia National Park: Gilmore Marsh near Aunt Betty Pond on Mount Desert Island and an unnamed beaver pond on the Schoodic Peninsula. The goal of this study was to relate habitat usage and diet to blood mercury concentrations in four riparian songbirds, the Common Yellowthroat Warbler, White-throated Sparrow, Song Sparrow, and Swamp Sparrow. We found that different species vary in their mercury levels and individuals of the same species vary in their mercury levels between sites, although these differences were not statistically significant. We also found that territory sizes differed for individuals at each site, which may be due to individual preferences, food availability, and competition. Diet analyses also showed that riparian songbirds are eating emergent aquatic insects, which may aid to the biomagnification of methylmercury within the food web. However, the results demonstrate that further research is necessary to determine relationships between habitat usage, diet, and mercury in riparian songbirds.