The Herring Gull is a year-round resident of the Great Lakes. Other fish-eating birds, such as terns, cormorants, herons and other gulls, migrate annually and can become contaminated while wintering elsewhere. This makes it difficult to determine exactly which contaminants in these species originated from the Great Lakes. Herring Gulls are common to the entire Great Lakes basin. They are found on all five lakes as well as in other parts of Canada and the world, allowing for comparison of contaminant levels between populations.
Wildlife species show a wide range in sensitivity to toxic chemicals.
As cleanup activities reduce the levels of toxic chemicals in the Great Lakes basin, some species become healthier sooner than others. The least sensitive species, tolerant of relatively high concentrations of contaminants, recover first. The most sensitive, affected at relatively low concentrations, recover last. Their recovery depends more on a cleanup of the entire ecosystem. The Herring Gull is probably the least sensitive of the species affected and has shown considerable recovery in terms of eggshell thickness and reproductive success, since the early 1970s.