Understanding the social dynamics of breeding phenology: Indirect genetic effects and assortative mating in a long-distance migrant

Abstract

Phenological traits, such as the timing of reproduction, are often influenced by social interactions between paired individuals. Such partner effects may occur when pairmembers affect each other’s prebreeding 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 lesser extent, genetic component of its 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 assortativemating leads to overestimation of partner effects.Our study provides evidence for partner effects on breeding phenology in a longdistance migrant while uncovering the potential causal pathways underlying the observed effects and raising awareness for confounding effects resulting from assortative mating or other common environmental effects.