Herring gulls (Larus argentatus) are used as ecological indicators of the coastal Lake Superior ecosystem in Pukaskwa National Park. Their populations have declined by 70% over the last 40 years, suggesting changes in the park ecosystem. I assessed herring gull diets and investigated how diet may influence population trends through effects on life-history endpoints. I additionally investigated the degree to which predation is affecting herring gull reproductive success by examining gull nest attentiveness with camera traps. Gulls utilizing anthropogenic food sources exhibited reduced levels of stress-associated hormones, increased egg size, and increased day-time nest attentiveness. Gulls were significantly more attentive to their nests during the day compared to the night. Nocturnal predation by great horned owls was the most significant predation-related factor affecting herring gull nest success. Understanding factors contributing to population trends in ecological indicator species is critical for identifying stressors that are likely affecting the broader ecosystem.