Following the discovery of widespread adverse reproductive effects in fish-eating colonial waterbirds nesting in the Canadian Great Lakes, Environment Canada started monitoring contaminants in Herring Gull (Larus argentatus) eggs in 1974. Current and historical concentrations and rates of decline of legacy contaminants (Polychlorinated Biphenyls [PCBs], 2,3,7,8-tetrachlorodibenzo-p-dioxin [TCDD] and organochlorine pesticides) in Herring Gull eggs from 15 Great Lakes colonies over 40 years are reported here. Large declines in contaminant concentrations were found in all colonies from the first year of reporting to 2013, with mean percent declines ranging from 72.7% for Σ chlordane to 95.2% for Mirex, indicating reduced availability of contaminants to wildlife. First-order exponential decay regressions indicated that rates of decline in eggs varied among compounds. Herring Gulls from Strachan Island (St. Lawrence River), for example, had the highest rates of decline for Dieldrin and Hexachlor Epoxide, whereas those from Middle Island (Lake Erie) had the lowest rates of decline for TCDD and PCBs, and those from Gull Island (Lake Michigan) had the lowest rates of decline for HCB and Mirex. Exponential rates of decline in Herring Gulls mirrored those previously reported for Great Lakes fish, thus demonstrating that Herring Gulls are a good fish-eating indicator species.