Differential breeding investment in bridled and non-bridled common guillemots (Uria aalge): morph of the partner matters

Abstract

Polymorphism is the coexistence of two or more phenotypically distinct and genetically determinate forms in a population and implies a selective balance between the alternative morphs to permanently exist. Common guillemots Uria aalge occur in two genetically distinct morphs—a non-bridled and a bridled—the latter with white eye rings and a well-defined stripe behind the eyes. In this study, we investigated differences between the morphs with regard to reproductive parameters. We used a detailed family-based sample providing data on mother, father, and chick over three breeding seasons. The mating between morphs was random but pure non-bridled and pure bridled pairs produced smaller chicks at age 15 days than the two mixed pair compositions. Body mass of the adults showed much the same pattern; pure pairs having lower body mass than mixed pairs. There were no differences between the two morphs in reproductive parameters without considering the morph of the partner. This suggests that reproductive decisions in some way not only depend on the morph but also on the tactic of the partner. Different reproductive strategies between morph family groups as found in this study may contribute to the understanding of the existence of a balanced polymorphism in common guillemots. The overall breeding conditions during the years of this study were good. However, over time in a variable environment, we suggest that tactics of different family groups may have different success stabilizing the frequency of non-bridled and bridled birds over time.