© Marcel Burkhardt
Altwegg, R., A. Roulin, M. Kestenholz & L. Jenni (2003)
Variation and covariation in survival, dispersal, and population size in barn owls Tyto alba.
J. Anim. Ecol. 72: 391–399
1. Population dynamics are the result of variation in survival, recruitment, emigration and immigration rates. Covariation between these demographic rates can affect the dynamics and extinction risk of a population at the local and regional scale. The source-sink and the balanced dispersal process, two processes hypothesised to govern emigration and immigration, are expected to lead to different patterns of covariation between demographic rates in local populations.
2. Here, we study variation and covariation in survival, emigration, immigration, and the size of a Swiss barn owl (Tyto alba) population, and examine the factors associated with this variation. For practical reasons, survival and emigration are often confounded in studies of natural populations. We overcome this problem by jointly analysing life-recapture and dead-recovery data using recently developed statistical methods.
3. Mortality, emigration, the number of known immigrants, and population size were positively correlated over time. Neither the source-sink nor the balanced dispersal process is expected to lead to this pattern.
4. Survival was lower for juveniles than adults (mean 17% vs. 72%), and highly variable across years in both age classes. Snow cover, mean annual temperature and population density accounted for 32% to 47% of the variation in survival of juveniles and adults. Emigration was higher for juveniles than for adults, and adults emigrated more often when the population size was high.
5. Our results showed that local population dynamics were affected by density dependence and the stochastic environmental factors snow cover and temperature. Rather than lending support to one of the two dispersal hypotheses, the patterns of correlation between survival, dispersal, and population size suggest strong fluctuations in environmental conditions that influence the dynamics of barn owl populations at the regional scale.