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1. A population's effective size (Ne) is a key parameter that shapes rates of inbreeding and loss of genetic diversity, thereby influencing evolutionary processes and population viability. However, estimating Ne and identifying key demographic mechanisms that underlie the Ne to census population size (N) ratio, remains challenging, especially for small populations with overlapping generations and substantial environmental and demographic stochasticity and hence dynamic age-structure. 2. A sophisticated demographic method of estimating Ne/N, which uses Fisher's reproductive value to account for dynamic age-structure, has been formulated. However, this method requires detailed individual and population-level data on sex and age-specific reproduction and survival and has rarely been implemented. 3. Here, we use the reproductive value method and detailed demographic data to estimate Ne/N for a small and apparently isolated red-billed chough (Pyrrhocorax pyrrhocorax) population of high conservation concern. We additionally calculated two single-sample molecular genetic estimates of Ne to corroborate the demographic estimate and examine evidence for unobserved immigration and gene flow. 4. The demographic estimate of Ne/N was 0.21, reflecting a high total demographic variance (σ 2 dg) of 0.71. Females and males made similar overall contributions to σ 2 dg. However, contributions varied among sex-age classes, with greater contributions from 3 year-old females than males, but greater contributions from ⩾5 year-old males than females. 5. The demographic estimate of Ne was ∼30, suggesting that rates of increase of inbreeding and loss of genetic variation per generation will be relatively high. Molecular genetic estimates of Ne computed from linkage disequilibrium and approximate Bayesian computation were approximately 50 and 30, respectively, providing no evidence of substantial unobserved immigration which could bias demographic estimates of Ne. 6. Our analyses identify key sex-age classes contributing to demographic variance and thus decreasing Ne/N in a small age-structured population inhabiting a variable environment. They thereby demonstrate how assessments of Ne can incorporate stochastic sex and age-specific demography and elucidate key demographic processes affecting a population's evolutionary trajectory and viability. Furthermore, our analyses show that Ne for the focal chough population is critically small, implying that management to re-establish genetic connectivity may be required to ensure population viability. [ABSTRACT FROM AUTHOR]

1. Understanding the degree to which reproductive success varies with an individual’s age and lifespan, and the degree to which population-level variation mirrors individual-level variation, is central to understanding life-history evolution and the dynamics of age-structured populations. We quantified variation in the survival probability of offspring, one key component of reproductive success and fitness, in relation to parent age and lifespan in a wild population of red-billed choughs ( Pyrrhocorax pyrrhocorax). 2. On average across the study population, the first-year survival probability of offspring decreased with increasing parent age and lifespan; offspring of old parents were less likely to survive than offspring of young parents, and offspring of long-lived parents were less likely to survive than offspring of short-lived parents. 3. However, survival did not vary with parent age across offspring produced by groups of parents that ultimately had similar lifespans. 4. Rather, across offspring produced by young parents, offspring survival decreased with increasing parent lifespan; parents that ultimately had long lifespans produced offspring that survived poorly, even when these parents were breeding at young ages. 5. The average decrease in offspring survival with increasing parent age observed across the population therefore reflected the gradual disappearance of short-lived parents that produced offspring that survived well, not age-specific variation in offspring survival within individual parents. 6. The negative correlation between offspring survival and maternal lifespan was strongest when environmental conditions meant that offspring survival was low across the population. 7. These data suggest an environment-dependent trade-off between parent and offspring survival, show consistent individual variation in the resolution of this trade-off that is set early in a parent’s life, and demonstrate that such structured life-history variation can generate spurious evidence of senescence in key fitness components when measured across a population. [ABSTRACT FROM AUTHOR]