Distribution of Robins Erithacus rubecula in wintering grounds: effects ofconspecific density, migratory status and age

This paper studies how population density affects the dynamics of habitat o ccupation by Robins in a wintering area of southern Spain (Gibraltar area), and how the between-habitat distribution is influenced by the age and migr atory behaviour of birds (whether they are year-round residents or winterin g migrants). We studied two different habitat types: forested sites, some o f which are breeding habitats of Robins, and shrubland sites, to which Robi ns spread their range in autumn-winter due to the arrival of migrants. As a rriving migrants increased abundance in the area, new habitat patches were sequentially colonised, forested sites being the first to be occupied and t he last to be abandoned (regardless they maintained breeding populations or not). In addition, the abundance of Robins tended to decrease along the au tumn-winter period in the most densely populated patches, which made the am ong-site distribution of Robins to be progressively more uniform. Juveniles of both local and migratory populations dominated (83% of individuals) in lowlands. Local Robins were more abundant (67%) than migratory ones in mont ane forests, although they also contributed to the winter colonisation of l owland habitats (47%). Together, these results support that the between-hab itat distribution of Robins depends not only on population density, but als o on the migratory status and age of individuals attempting to get a winter ing site. Therefore, if winter habitats differed in quality, these factors could influence the mechanisms of regulation of Robin populations in the no n-breeding season.