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Consequences of heterogeneity in survival probability in a population of Florida scrub-jays

Authors

  • GORDON A. FOX,

    1. Department of Biology (SCA 110), University of South Florida, Tampa, FL 33620, USA; Donald Bren School of Environmental Science and Management, University of California Santa Barbara, Santa Barbara, CA 93106-5131, USA;
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  • BRUCE E. KENDALL,

    1. Department of Biology (SCA 110), University of South Florida, Tampa, FL 33620, USA; Donald Bren School of Environmental Science and Management, University of California Santa Barbara, Santa Barbara, CA 93106-5131, USA;
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  • JOHN W. FITZPATRICK,

    1. Cornell Laboratory of Ornithology, Cornell University, 159 Sapsucker Woods Road, Ithaca, NY 14850, USA; and
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  • GLEN E. WOOLFENDEN

    1. Archbold Biological Station, PO Box 2057, Lake Placid, Florida 33862, USA
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Gordon A. Fox, Department of Biology (SCA 110) and Department of Environmental Science & Policy, University of South Florida, Tampa, FL 33620, USA. Fax: +1 813 974 3263. E-mail: gfox@mail.cas.usf.edu

Summary

  • 1Using data on breeding birds from a 35-year study of Florida scrub-jays Aphelocoma coerulescens (Bosc 1795), we show that survival probabilities are structured by age, birth cohort, and maternal family, but not by sex. Using both accelerated failure time (AFT) and Cox proportional hazard models, the data are best described by models incorporating variation among birth cohorts and greater mortality hazard with increasing age. AFT models using Weibull distributions with the shape parameter > 1 were always the best-fitting models.
  • 2Shared frailty models allowing for family structure greatly reduce model deviance. The best-fitting models included a term for frailty shared by maternal families.
  • 3To 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.
  • 4The data are not adequate for testing hypotheses as to whether variation in frailty is heritable.
  • 5Substantial 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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