Individual heterogeneity and senescence in Silvereyes on Heron Island

Research output: Contribution to journalArticle


Individual heterogeneity and correlations between life history traits play a fundamental role in life history evolution and population dynamics. Unobserved individual heterogeneity in survival can be a nuisance for estimation of age effects at the individual level by causing bias due to mortality selection. We jointly analyze survival and breeding output from successful breeding attempts in an island population of Silvereyes (Zosterops lateralis chlorocephalus) by fitting models that incorporate age effects and individual heterogeneity via random effects. The number of offspring produced increased with age of parents in their first years of life but then eventually declined with age. A similar pattern was found for the probability of successful breeding. Annual survival declined with age even when individual heterogeneity was not accounted for. The rate of senescence in survival, however, depends on the variance of individual heterogeneity and vice versa; hence, both cannot be simultaneously estimated with precision. Model selection supported individual heterogeneity in breeding performance, but we found no correlation between individual heterogeneity in survival and breeding performance. We argue that individual random effects, unless unambiguously identified, should be treated as statistical nuisance or taken as a starting point in a search for mechanisms rather than given direct biological interpretation.


  • Jonas Knape
  • Niclas Jonzén
  • Martin Sköld
  • Jiro Kikkawa
  • Hamish McCallum
Research areas and keywords

Subject classification (UKÄ) – MANDATORY

  • Biological Sciences


  • breeding, individual heterogeneity, parameter identifiability, senescence, Silvereye, survival, Zosterops lateralis chlorocephalus
Original languageEnglish
Pages (from-to)813-820
Issue number4
Publication statusPublished - 2011
Publication categoryResearch