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. Previous studies highlighted declining prey abundance as a possible contributing factor to population declines. Populations of avian predators that prey on herring gulls have simultaneously increased which could be a significant factor impacting gull population trends. Here, I assessed herring gull diets via stable isotope (nitrogen, carbon) and fatty acid indicators to investigate how diet may influence population trends through effects on physiological (stress-associated hormones), reproductive (egg size), and behavioural (nest attentiveness) endpoints. I additionally investigated the degree to which predation is affecting herring gull reproductive success by examining gull nest attentiveness during the day and night. Gulls utilizing anthropogenic food sources exhibited reduced levels of stress-associated hormones, increased egg size, and increased day-time nest attentiveness. Anthropogenic food sources are likely buffering the impacts of declines in aquatic food availability; however, gull populations are still declining. Use of camera traps to monitor herring gull nests revealed that gulls were significantly more attentive to their nests during the day compared to the night. Of the nine nests monitored with camera traps, five produced chicks. However, at three of these nests chicks were predated shortly after hatch. Nocturnal predation by great horned owls accounted for these predation events which were the most significant predation-related factor affecting herring gull nest success. Predation of chicks at night and of eggs during the day (by conspecifics and corvids) is likely contributing to declines in PNP herring gull populations. Understanding factors contributing to population trends in ecological indicator species is critical for species management and for identifying stressors that are likely