The diets of four seabird species (N = 20–21 individuals per species) were compared to determine whether sexual size dimorphism is involved in intersexual differences in diet composition and niche size. Diet compositions of Black-legged Kittiwake (Rissa tridactyla), Herring Gull (Larus argentatus), Great Black-backed Gull (Larus marinus) and Razorbill (Alca tarda) were assessed during their breeding season in the Gulf of St. Lawrence, Canada by using: Bayesian mixing models using stable carbon and nitrogen isotope analysis on the red blood cells of adults and direct field assessments of food items carried by adults to chicks. A total of 50 regurgitations for kittiwakes, 40 regurgitations and 76 pellets for gulls (two species combined) and 78 prey loads for Razorbills were characterized. Diet composition varied widely among seabird species and between methods of assessment. Isotopic niche size of adults was greater for males than for females for species with clear dimorphism (Herring Gull and Great Black-backed Gull), whereas differences in niche size between sexes became insignificant when sexual size dimorphism decreased (Black-legged Kittiwake). Conversely, for the monomorphic Razorbill, females displayed a larger isotopic variance than males, suggesting that factors other than body size are involved in shaping niche size.