The quantitative study of marked individuals in ecology, evolution ...

EURING 2003 Radolfzell
the effects of spring snow cover and a spring conservation hunt on breeding propensity
using a weighted least squares approach. We also used an empirical variancecomponents
approach and determined that true temporal variation in breeding propensity
was considerable (mean breeding propensity: 0.574 [95% CI considering only
process variation: 0.13 to 1]). Spring snow cover was negatively related to breeding
propensity (βsnow = -2.05 ± 0.96 SE) and tended to be reduced in years with a spring
hunt (β = -0.78 ± 0.35). Nest densities on the breeding colony and fall young/adult ratio
were good indices of breeding propensity, with nest densities being slightly more
precise. These results suggest that environmental conditions and disturbance encountered
during the pre-breeding period can have a significant impact on productivity
of Arctic-nesting birds.
09:20 - 09:45 AM
Earlier recruitment or earlier death? On assumptions of homogeneous survival rates
in capture-recapture models to estimate recruitment
Emmanuelle Cam, Evan Cooch & Jean-Yves Monnat
Realized patterns of age of recruitment observed in the breeding segment of populations
are governed by the product of two demographic components: [survival probability
from birth to age ]*[transition probability from state prebreeder to state first-time
breeder at age ]. To again insight into selective pressures shaping age of recruitment,
one may address temporal variation in age of first breeding and covariation with population
size or social and environmental factors. This involves comparison among
"groups" (e.g., cohorts) of individuals encountering different environmental conditions
when they reach a given age as prebreeders.
However, measures of recruitment based exclusively on data from the breeding segment
of the population ignore the size of the pool of prebreeders (as opposed to transition
probability). However, in a large number of species with deferred breeding, individuals
are not encountered prior to breeding. Approaches to estimating age-specific
recruitment probability in the absence of data from prebreeders using mark-recapture
have been developed (Pradel and Lebreton 1999, Schwarz and Arnasson 2001, Williams
et al. 2002). Unless the survival component of realized age-specific recruitment
rates is known to be identical among the groups compared, inferences about the
"cause" of variation in realized age of first breeding among groups are difficult: such
differences may in fact result from differences in survival probability before individuals
make the transition between states. Inferences about differences in realized recruitment
rates among groups exclusively based on data from the breeding segment of
the population reflect differences in recruitment probability under the assumption that
there is no difference in prebreeder survival among these groups.
We assessed the consequences of violations of this assumption on our perception of
age-specific realized recruitment rates using numerical simulations; the scenarios
considered correspond to various biological hypotheses about group-specific variation
in survival and transition probabilities. Data were simulated under regular multistate
models (Nichols and Kendall 1995) and truncated individual histories were then analyzed
using the reverse-time approach (Pradel 1996) and the approach developed by
Schwarz and Arnasson (2001). Depending on the scenario, realized recruitment was
delayed or advanced compared to the underlying pattern of age-specific transition
probabilities. We also addressed age-specific recruitment probability in a long-lived
seabird species, the kittiwake (Rissa tridactyla) using a data set including data from
prebreeders. We compared results from the reverse-time approach and the multistate
approach, and showed that transition probability directly estimated or derived from
7