Résumé
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.