Heterogeneity within a population is a pervasive challenge for studies of individual life-histories. Population-level patterns in agespecific reproductive success can be broken down into relative contributions from selective disappearance, selective appearance of individuals into the study population, and average change in performance for survivors (average ontogenetic development). In this article, we provide an exact decomposition. We apply our formula to data on the reproductive performance of a well characterized population of common terns (Sterna hirundo). We show that improvements with age over most of adult life and senescence at old ages are primarily due to a genuine change in the mean among surviving individuals rather than selective disappearance or selective appearance of individuals. Average ontogenetic development accounts for approximately 87% of the overall age-specific population change.common tern (Sterna hirundo) | age-specific reproduction | aging | heterogeneity | exact decomposition approach I n the study of life-histories it is important to capture the performance of individuals and changes in that performance over age. Changes in performance are often reported as changes in population averages, but individual heterogeneity can produce dynamics at the observed population level that are very different from dynamics at the individual level (1-3)."Population level" refers to the observed averaged information over all individuals present at a given age regardless of whether they survive to the next age step. What we define as population change is the change observed directly in the aggregate field data and not the change in a hypothetical population of all individuals that were present at a given starting age x. "Individual level" refers to the change over age for each surviving individual averaged over all these survivors. We call this term "average ontogenetic development."The interpretation of population-level patterns can be problematic because of "within-generation phenotypic selection" (4). This is a change in the composition of the population if selective mortality removes frail (of lower quality) individuals at an earlier age than those individuals that are less frail (of higher quality) (1, 2), or if individuals that enter the population (here breeding population) as adults later than others through delayed recruitment or immigration (5) have a different performance than the resident population. We use the phrase "selective disappearance" to denote change in the mean of a phenotypic trait due to mortality and "selective appearance" to denote change due to new appearance of individuals. The term "compositional change" is used to refer to the combination of both selective appearance and disappearance. We develop a method to exactly decompose population change in an age-specific phenotypic trait with repeated measures into the components: average ontogenetic development, selective disappearance, and selective appearance.An Approach to Decomposing Population Change. If there is no selecti...

Summary1. Understanding age-specific survival in wild animal populations is crucial to the study of population dynamics and is therefore an essential component of several fields including evolution, management and conservation. 2. We present Bayesian survival trajectory analysis (BaSTA), a free open-source software package for estimating age-specific survival from capture-recapture/recovery data under a Bayesian framework. 3. The method copes with low recapture probabilities, unknown ages (e.g. because of left-truncation) and unknown ages at death (e.g. because of right-censoring). It estimates survival and detection parameters as well as the unknown birth and death times (i.e. latent states) while allowing users to test a range of survival models. In addition, the effect of continuous or categorical covariates can be evaluated. 4. This tool facilitates the analysis of age patterns of survival in long-term animal studies and will enable researchers to robustly infer the effect of covariates, even with large amounts of missing data.

Summary 1.Reproductive value is an integrated measure of survival and reproduction fundamental to understanding life-history evolution and population dynamics, but little is known about intraspecific variation in reproductive value and factors explaining such variation, if any. 2. By applying generalized additive mixed models to longitudinal individual-based data of the common tern Sterna hirundo, we estimated age-specific annual survival probability, breeding probability and reproductive performance, based on which we calculated age-specific reproductive values. We investigated effects of sex and recruitment age (RA) on each trait. 3. We found age effects on all traits, with survival and breeding probability declining with age, while reproductive performance first improved with age before levelling off. We only found a very small, marginally significant, sex effect on survival probability, but evidence for decreasing age-specific breeding probability and reproductive performance with RA. 4. As a result, males had slightly lower age-specific reproductive values than females, while birds of both sexes that recruited at the earliest ages of 2 and 3 years (i.e. 54% of the tern population) had somewhat higher fitness prospects than birds recruiting at later ages. While the RA effects on breeding probability and reproductive performance were statistically significant, these effects were not large enough to translate to significant effects on reproductive value. 5. Age-specific reproductive values provided evidence for senescence, which came with fitness costs in a range of 17-21% for the sex-RA groups. 6. Our study suggests that intraspecific variation in reproductive value may exist, but that, in the common tern, the differences are small.

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