Phenological traits, such as the timing of reproduction, are often influenced by social interactions between paired individuals. Such partner effects may occur when pair members affect each other’s pre-breeding environment. Partner effects can be environmentally and/or genetically determined, and quantifying direct and indirect genetic effects is important for understanding the evolutionary dynamics of phenological traits. Here, using 26 years of data from a pedigreed population of a migratory seabird, the common tern ( Sterna hirundo ), we investigate male and female effects on female laying date. We find that female laying date harbors both genetic and environmental variation, and is additionally influenced by the environmental, and, to a lower extent, genetic, component of her mate. We demonstrate this partner effect to be largely explained by male arrival date. Interestingly, analyses of mating patterns with respect to arrival date show mating to be strongly assortative and, using simulations, we show that this assortative mating for arrival date leads to overestimation of partner effects. Our study provides evidence for partner effects on breeding phenology in a long distance migrant, while uncovering the potential causal pathways underlying the observed effects and raising awareness for confounding effects due to assortative mating or other common environmental effects.