Variation and covariation in survival, dispersal, and population size in barn owls Tyto alba

1. Population dynamics are the result of variation in survival, recruitment, emigration and immigration rates. Covariation between these demographic rates critically affects the dynamics and magnitude of fluctuations in population size. Such covariation can arise by environmental factors affecting several rates in similar ways. Also demographic processes, such as the source-sink and the balanced dispersal process, are expected to lead to covariation between demographic rates. 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 live-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 32-47% 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 higher. 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 a larger spatial scale.