Heterogeneity in individual quality in birds: overall patterns and insights from a study on common terns

Abstract

While life‐history theory predicts a tradeoff between reproduction and survival, positive covariance, indicative of heterogeneity in individual quality, is often reported among individuals from natural populations. We review longitudinal studies of wild bird populations that test the relationship between annual reproductive success and lifespan and find the majority to report a positive correlation, while none reports a negative correlation. Heterogeneity in individual quality in resource acquisition, masking resource‐based tradeoffs, therefore appears to be common in birds. Considering that there is little evidence for heritable variation in fitness, heterogeneity in individual quality among adults may be due to life‐long effects of developmental conditions. In a 20‐year case study on common terns Sterna hirundo, we test for life‐long effects of cohort quality and within‐cohort nest quality, but find no significant effects on long‐term proxies of quality. Since other studies do find strong life‐long effects of developmental conditions, we suggest that the brood reduction strategy adopted by common terns, causing the majority of offspring to die rapidly after hatching, efficiently reduces variation in offspring quality at independence. As such, a brood reduction strategy may contribute to reduced heterogeneity in adult survival in stochastic environments, both suggested to be more common and adaptive in long‐lived species. Further study is required to assess heterogeneity in individual reproduction, especially in relation to environmental stochasticity and species’ life‐history strategies, in order to assess whether the relative strength of selection in early and late life may indeed affect the magnitude of heterogeneity in individual quality over life, and how this is mediated by parent–offspring conflict.