Nearly unbiased estimator of adult population size based on within-cohort half-sibling pairs incorporating flexible reproductive variation

Close-kin mark-recapture (CKMR) is a kinship-based method for estimating adult abundance. However, the application of CKMR is limited to using a kinship relationship that is not affected by family-correlated survivorship, which leads to a biased estimation. We developed a nearly unbiased estimator of the number of mothers in a population, which is based on the known maternal half-sibling relationship found within the same cohort. Our method allowed for variance of the averaged offspring number per mother (between-age variation) and for variance of the offspring number among mothers with the same reproductive potential (within-age variation). Estimators of its variance and coefficient variation were also provided. The performance of the estimators was quantitatively evaluated by running an individual-based model. Our results provide guidance for (i) a sample size to archive the required accuracy and precision when the order of mother size is available and (ii) a degree of uncertainty regarding the estimated mother size when information about the mother size is not available. Taken together, these find- ings offer an opportunity to shed light on the usefulness of analysing within-cohort half-sibling pairs and will greatly widen the scope of the CKMR method.