Exploring density‐dependent relationships in demographic parameters in populations of birds at a large spatial scale

Abstract

The importance of density‐dependent processes in natural populations is widely accepted, but the issue of the shape of density‐dependent relationships (such as influenced by vagueness, or time‐delay) remains unresolved. We explored the density‐dependent relationships in demographic parameters for 12 species of birds in Britain using large‐scale, long‐term data sets. We predicted that a negative relation between density and demographic parameters should be observed for the stable species, whereas the decreasing or increasing species should display a positive relation if the environment changes progressively through time bringing about a continuous change in density dependence. Our prediction was verified for nine species out of 12; however, we observed, for the three remaining species, a significant decrease of survival rates through time that seems to be involved in a long‐term population decline. In all cases where a density‐dependent relation was found, we observed an important variance around the relation. In one case, we showed that this variance increased significantly with density. We found evidence for time‐delayed effects of density dependence both for survival and breeding performance. In two species, our results suggest the existence of complex interactions (compensatory mechanisms) between survival and breeding performance or between the different components of breeding performance.