Understanding the Impact of Correlation within Pair-Bonds on
Cormack-Jolly-Seber Models
Abstract
1. The Cormack-Jolly-Seber (CJS) model and its extensions have been
widely applied to the study of animal survival rates in open
populations. The model assumes that individuals within the population of
interest have independent fates. It is, however, highly unlikely that a
pair of animals which have formed a long-term pairing have dissociated
fates. 2. We examine a model extension which allows animals who have
formed a pair-bond to have correlated survival and recapture fates.
Using the proposed extension to generate data, we conduct a simulation
study exploring the impact that correlated fate data has on inference
from the CJS model. We compute Monte Carlo estimates for the bias,
range, and standard errors of the parameters of the CJS model for data
with varying degrees of survival correlation between mates. Furthermore,
we study the likelihood ratio test of gender effects within the CJS
model by simulating densities of the deviance. Finally, we estimate the
variance inflation factor for CJS models that incorporate sex-specific
heterogeneity. 3. Our study shows that correlated fates between mated
animals may result in underestimated standard errors for parsimonious
models, significantly deflated likelihood ratio test statistics, and
underestimated values of the variance inflation factor for models taking
sex-specific effects into account. 4. Underestimated standard errors can
result in lowered coverage of confidence intervals. Moreover, deflated
test statistics will provide overly conservative test results. Finally,
underestimated variance inflation factors can lead researchers to make
incorrect conclusions about the level of extra-binomial variation
present in their data.