Monogamy with exceptions: Social and genetic mating system in a bird species with high paternal investment

Animal mating systems are strongly linked to costs and benefits associated with parental care. A high degree of male parental care may explain the occasional occurrence of social polyandry in some monogamous birds such as woodpeckers. We examined mating and parental behaviours of 55 three-toed woodpeckers Picoides tridaclylus, and used DNA fingerprinting to study its genetic mating system over 11 years. Sexes shared nest hole guarding and parental care, but males allocated significantly more time to territory defence, cavity excavation and feeding young than females. Two (7.4%) of the 27 females were polyandrous, each paired simultaneously with two males. Four (7.3%) out of 55 chicks were extra-pair (15.4% of the broods). Two of these chicks were associated with social polyandry, and one with female parasitism. Hence, three-toed woodpeckers were socially and genetically predominantly monogamous. We propose that both social and genetic polyandry in woodpeckers are constrained by the long duration of cavity excavation, few re-mating opportunities, and by the fact that bi-parental care is important for reproductive success.