Effect of Senescence on Estimation of Survival Probability When Age Is Unknown

Adult survival probability is a key parameter in any population model for a long-lived species. For many species, information on adult survival comes from a capture–recapture study involving individuals for whom age is unknown. If the species experiences senescence, the estimate of overall adult survival probability will be negatively biased. The purpose of this paper is to assess the extent to which the estimate is biased and the implications for population modelling. We show that the amount of bias depends on the capture probability and the strength of senescence. If the capture probability is greater than 0.5, the expected bias is at most 1%, unless senescence is strong and begins early in adulthood. Individual heterogeneity in capture probability can also lead to negative bias in estimates of survival probability, meaning that moderate effects from senescence and heterogeneity may combine to produce a non-negligible amount of bias. Capture–recapture methods for survival are also used to estimate the time that migrating animals spend at intermediate “stop-over” sites. In this context, an increase in departure probability with time since arrival is analogous to senescence, leading to a negative bias in estimated stop-over duration. This bias will often be large because capture probabilities in such studies are generally very low and departure probability may increase abruptly once animals have rested and re-fueled.