Spatial trends in levels of biomagnifying environmental contaminants in tissues of top predators can provide insights into potential contaminant sources and dynamics. I use gull eggs to elucidate spatial trends in environmental availability of mercury in western Canada. I begin by validating the use of eggs as a matrix for monitoring mercury bioavailability through an experimental laboratory study. Next, I investigate mercury spatial trends in wild gull eggs collected at twelve sites located across 14 degrees of latitude. Assessing levels of biomagnifying contaminants can be confounded by dietary variability, thereby I apply amino acid-compound specific stable nitrogen isotope analysis (AA-CSIA) to generate trophic position-adjusted mercury levels that are more suitable for spatial comparisons. Spatial differences in egg mercury levels were evident with highest values at sites in receiving waters of Athabasca River. My research demonstrates the utility of the AA-CSIA approach in enhancing our interpretation of contaminant monitoring data.