The zone of secondary contact between Herring Gulls (Larus argentatus) and Caspian Gulls (L. cachinnans) in central Poland is characterized by high levels of introgreeRzemyscular mars several phenotypic characters, with the exception of bare-parts coloration. These patterns suggest a role for coloration as a premating isolating barrier between these species. We investigated the degree of reproductive isolation in this zone of secondary contact by testing three hypotheses: (1) postmating isolation is weak or absent, (2) a partial premating isolation is present, and (3) orbital-ring color or another closely correlated trait is involved in premating isolation, which explains the signal of disruptive selection previously observed (a lower degree of introgression). We also investigated whether differences in breeding phenology could contribute to premating isolation. We found no evidence of postmating isolation, but premating isolation occurred: heterospecific pairs were significantly scarcer than expected under random mating. This assortative mating is best explained by a combination of divergent breeding phenology and mate choice influenced by orbital-ring color. However, because the differentiation in orbital-ring color between L. argentatus and L. cachinnans is incomplete and their breeding periods overlap, premating isolation is only partial. That a previous study found a high level of genetic introgression between these species suggests that reinforcement can occur only in association with sufficiently strong postmating isolation.