Vital rates of intermittent nonbreeders and returning breeders strongly influence population dynamics of Somateria mollissima (Common Eider)

Abstract

Risks and costs associated with reproduction may induce iteroparous species to employ intermittent breeding, whereby individuals forgo attempting reproduction in some breeding periods following recruitment to the breeding population. We explore this behavior through population-level analyses of Somateria mollissima (Common Eider), quantifying the survival cost of breeding and hence one benefit of skipping. Nonbreeding in a given year may be a short-term response to expected low fitness returns, avoiding either breeding-induced mortality or low offspring value. Alternatively, or additionally, intermittent breeding may be a long-term strategy maximizing lifetime fitness, with breeding trajectories that include nonbreeding years resulting in more recruited offspring over the whole life course than those with the same number of consecutive breeding attempts. Reanalysis of 3 studies reporting annual mortality schedules for S. mollissima allowed estimation of the proportion of mortality incurred during the peak breeding season (~50%) and hence the difference in survival rates between breeders and nonbreeders. These were incorporated into a life cycle and associated matrix population model with a “refreshed breeder” stage to which individuals transition for the time step following nonbreeding. We show that the transition to this stage strongly influences population growth rate—being more than twice as important as reproduction by continued breeders—mostly driven by the possibility of differential survival after skipping breeding. Our results emphasize the benefits of long-term individual-based studies that can identify refreshed breeders to further our understanding of intermittent breeding. Specifically, the accuracy of population projections could be improved by factoring in transitions following skipped breeding, and management enhanced by interventions facilitating return to the breeding pool, such as nest shelter provision