Craig Hebert and colleagues at the Canadian Wildlife Research Center in Ottawa analyzed 25 years of data on the gulls and discovered the Great Lakes birds are in poor health in many areas.

The scientists said fish are the gulls' staple diet, so it would be natural to assume more fish would mean better dining. Instead, the researchers said the addition of species such as exotic salmon hasn't been good for the birds. Tests of their tissues showed an increased in transfats associated with food produced by humans, suggesting the birds have been forced to make a shift from fish to terrestrial food, including garbage, Hebert said.

Experts aren't sure why the birds are eating more garbage when exotic salmon and trout are added to the waters but Herbert said the birds might be out-competed for their favorite prey of smaller fish. And when fish are unavailable, the birds turn to land for their foraging.

The study appeared in the April issue of the journal Ecology.