The Double-crested Cormorant (Phalacrocorax auritus) population in the North American Great Lakes region has increased substantially during the past three decades. Cormorants are likely recovering after a period of human-caused suppression; however, their resurgence has raised concerns regarding potential negative interactions with other waterbirds. To examine this issue, we compared measures of nesting success and aggressive behavior in Herring Gulls (Larus argentatus) nesting on two artificial islands in Hamilton Harbour, western Lake Ontario. Only breeding Herring Gulls occupied the first island, whereas a mixed colony of both Herring Gulls and cormorants occupied the second. Over three years, Herring Gulls in the single-species colony had greater nesting success than those in the colony shared with cormorants. Gulls in the mixed colony had more nests that contained cormorant eggs in addition to Herring Gull eggs, or that were usurped by cormorants. Analysis of individual Herring Gull egg fates revealed lower hatching rates and more eggs displaced from nests in the mixed colony. Due to the presence of cormorants, gulls in the mixed colony engaged in more aggressive interactions near their nests than those in the single-species colony. We conclude that at our study site cormorants were competing directly with Herring Gulls for nest sites and nesting material, and contributing to reduced Herring Gull reproductive success in the mixed colony. Our results suggest that interactions between species in mixed colonies may be more subtle than the displacement of breeding adults, and may not be detected by examining nest counts alone. We suggest that further studies examine cormorant interactions with other colonial bird species at additional sites.