Mercury levels in the environment have been increasing steadily since the industrial age. Mercury can have deleterious effects on a variety of types of tissue in vertebrates; however, effects of mercury in some animals and/or tissues may be masked by animal's compensatory mechanisms. It is still widely unknown how these rising levels are affecting wildlife, specifically Eastern Bluebirds (Sialia sialis). The goal of this research was to explore a novel method through which mercury may be affecting the fitness of Eastern Bluebirds exposed to varying levels of environmental mercury. Birds on contaminated and reference sites were monitored for two years and various fitness measures were recorded. Four different feather types were also taken to measure mercury's effects on structural coloration. This is the first study to show that mercury in feathers may impact plumage coloration through three possible mechanisms: (1) binding to the sulfide bonds in keratin causing a change in the regular spacing that produces the blue color, (2) inhibition of melanin synthesis, or (3) impacts on the overall health of individuals resulting in lower investment in color production. The overall effects of mercury on bluebirds' fitness and their offspring varied based on adult sex; females with high feather mercury had fewer eggs, and males with high blood mercury fledged fewer offspring. In conclusion, these studies showed a novel way in which mercury may impact birds in an exposed environment as well as ways in which mercury may affect fitness based on sex of the individual and time of exposure. / Master of Science

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