Mercury (Hg) is an element naturally found both in the environment, and released through a number of industrial processes. Importantly, Hg is a toxin known to cause a wide variety of effects from changes in endocrine and neurological pathways, to impaired reproduction and decreased survival. Avian species are widely used as indicators of environmental Hg contamination as they are sensitive to its effects and multiple endpoints can be easily assessed.My thesis aims to review and explore the pathways and effects of Hg on Arctic marine birds. First, I review the current levels, known effects and trends in eight marine bird species in northern Canada. Following this review I use a model avian species (the common eider; Somateria molissima) to better understand how Hg may affect marine birds independently and jointly with our other environmental stressors. In Chapter 2 I examine how sex, diet and individual size can influence both Hg and parasites as they share common host pathways. My results indicate that several different extrinsic factors can influence parasite and Hg distributions within a population, emphasizing the importance in assessing these factors when examining contaminant burdens in wildlife. In Chapter 3 I present an experimental manipulation of wild female eider ducks used to examine the effects of Hg on reproduction and survival under natural and reduced parasite burdens. My findings indicate parasites, but not Hg burdens, influence breeding decisions in this species. In Chapter 4 I investigate how Hg and lead (Pb; another natural metal known to have neurotoxic effects on wildlife) are associated directly and indirectly with known breeding pre-cursors (immunoglobulin Y, corticosterone levels, arrival date and arrival condition) in female eider ducks. While female eider ducks were found to have low levels of both Hg and Pb, I found that both varied significantly with the physiological and phenological indices examined indicating that these contaminants do influence breeding in marine birds even at levels often considered below toxic thresholds. My thesis results demonstrate that to fully understand how Hg affects wildlife, both direct and indirect effects must be considered throughout the annual cycle.