Bias and Loss of Precision Due to Tag Loss in Jolly–Seber Estimates for Mark–Recapture Experiments

A crucial, though often ignored, assumption of mark–recapture experiments is that animals do not lose their marks (tags). We present results of theoretical analyses of the effects of tag loss on estimates of population size (), survival (), births or new entries (), and on their standard errors (SE()), for the Jolly–Seber (full) model allowing birth and death. We show that, SE() and SE() are not biased by tag loss, while , , and SE() are biased. A similar analysis for the Jolly–Seber (death-only) model where births are known not to occur shows that , , and SE() are strongly biased by tag loss while only SE() is unbiased. Moreover, for both models, tag loss causes a loss in precision in all estimates (i.e. an increase in the standard error of the estimate, leading to wider confidence intervals). Throughout the paper, we assume that tag loss is homogeneous among animals; that is, it is the same for all marked animals regardless of age, sex, or tag-retention time, although the rate per unit time may change o...