Herring Gull (Larus argentatus) populations in Pukaskwa National Park have declined by 70% over the last 40 years. Populations of avian predators that prey on Herring Gulls have increased which could be a significant factor impacting gull populations. Here, we investigate Herring Gull daytime and nighttime nest attentiveness at locations with and without evidence of nocturnal predators. In 2017, Herring Gull nest attentiveness was examined at two sites using remote cameras. At one of those sites Great Horned Owl (Bubo virginianus) predation was observed, and gull nighttime nest attentiveness was lower there than at the site where owls were not observed. There were no inter-site differences in daytime nest attentiveness. In 2018, Herring Gull nest attentiveness was further investigated at the site where owls were present. At that site, Herring Gull nighttime nest attentiveness was significantly lower than during the day. Extended periods of absence of gulls from their nests during the night corresponded with the presence of owls. Predation of nest contents, in addition to the effects of other environmental stressors, are likely contributing to declines in Pukaskwa National Park's Herring Gull population.