Abstract
Organochlorine contaminants were measured in pooled egg samples of colonial waterbirds, red-winged blackbirds and tree swallows breeding in or near Severn Sound, Lake Huron, an area designated by the International Joint Commission as an Area of Concern, during 1991. Breast muscle samples of staging waterfowl were also collected the preceding autumn. PCBs, DDE and mirex were the most prevalent organochlorine contaminants present in eggs (concentration ranges of 0.065 to 5.452, 0.118 to 3.12 and 0.007 to 0.174 mg/kg, respectively), while oxychlordane, cis-chlordane, dieldrin, heptachlor epoxide, trans-nonachlor, cis-nonachlor, OCS, HCB and beta-HCH were also detectable at low levels in some egg samples. Tetra- and pentachlorobenzene, and alpha-and gamma-HCH were not detected in any of the samples. Generally, the following interspecific gradient in contaminant levels in bird eggs was found: herring gulls = Caspian terns > common terns > tree swallows > blackbirds. Concentrations in the eggs of piscivorous birds in Severn Sound were similar or slightly lower than those from other colonies in Georgian Bay and Lake Huron, and considerably lower than those from colonies in Hamilton Harbour, Lake Ontario. Mirex levels in herring gulls and Caspian terns from Severn Sound colonies were an exception in that they were substantially higher than levels from other Lake Huron colonies (0.174 and 0.167 mg.kg-1 for the two species, respectively, versus <0.12 mg.kg-1), although still much lower than those from Lake Ontario (0.50 and 0.719, respectively). It is probable that mirex was accumulated by gulls and terns during winter or migrational stopovers on Lake Ontario. Low contaminant levels in common terns may have been a result of their consumption of a smaller size class of prey fish. Higher contaminant levels in tree swallow eggs in comparison to red-winged blackbird eggs may have been due to the preponderance of aquatic insects in their diets, as opposed to the larger proportion of terrestrial arthropods in blackbird diets. Only PCBs and DDE were detected in the breast muscle of lesser scaup and mallards.