Consequences of Heterogeneity in Survival Probability in a Population of Florida Scrub-Jays

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demographic stochasticity, demography, frailty, heritability, longevity

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1. Using data on breeding birds from a 35-year study of Florida scrub-jays Aphelocoma coerulescens (Bosc 1795), we show that survival probabilities are tructured by age, birth cohort, and maternal family, but not by ex. Using both accelerated failure time (AFT) and Cox proportional hazard models, the data are best escribed by models incorpo-rating variation among birth cohorts and greater mortality hazard with increasing age. AFT models using Weibull distributions with the shape arameter > 1 were always the best-fitting models. 2. Shared frailty models allowing for family structure greatly reduce model deviance. The best-fitting models included a term for frailty shared by maternal families. 3. To ask how long a data set must be to reach qualitatively the same conclusions, we repeated the analyses for all possible truncated data sets of 2 years in length or greater. Length of the data set affects the parameter estimates, but not the qualitative conclusions. In all but three of 337 truncated data sets the best-fitting models pointed to same conclusions as the full data set. Shared frailty models appear to be quite robust. 4. The data are not adequate for testing hypotheses as to whether variation in frailty is heritable. 5. Structured heterogeneity for survival exists in this population. Such structured heterogeneity has been shown to have substantial effects in reducing demographic stochasticity.

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Journal of Animal Ecology, v. 75, issue 4, p. 921-927