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Great Black-backed Gulls Larus marinus and American Herring Gulls L. argentatus smithsonianus are considered dietary generalists at the population level: they consume a variety of prey types, including the adults and offspring of other seabirds, and they shift among prey types as availability varies. Atlantic Puffin Fratercula arctica adults, eggs, and chicks are often the target of depredation, and adults are also susceptible to kleptoparasitic attacks by large gulls during their summer breeding season in coastal Newfoundland, Canada. We investigated the depredation and kleptoparasitism of puffins by large gulls during July-August 2021/22 at a multi-species breeding colony on the northeast coast of Newfoundland. We hypothesized that both the inshore arrival of spawning Capelin Mallotus villosus, a key forage fish species in the region, and the onset of the puffin chick-rearing period, which typically coincide, influence puffin depredation and kleptoparasitism by large gulls. Shoreline transects on James Island revealed a decrease in new adult puffin carcasses during puffin chickrearing and Capelin spawning, with most adult puffin depredation occurring before the inshore arrival of spawning Capelin in both years. Behavioural observations (i.e., focal animal, behavioural sampling, and instantaneous scan sampling) revealed that Great Black-backed Gulls were more often present within puffin burrow areas and spent a higher proportion of time in these areas relative to American Herring Gulls, especially during Capelin spawning and puffin chick-rearing. Within puffin burrow areas, Great Black-backed Gulls depredated chicks and kleptoparasitized adults. As only ~30% of followed individuals of both gull species entered puffin burrow areas, it is possible that only a few Great Black-backed Gulls were responsible for puffin depredation and kleptoparasitism. [ABSTRACT FROM AUTHOR]