Why are migrant Robins (Erithacus rubecula) territorial in winter?: the importance of the anti-predatory behaviour

Abstract

The functional explanation of nonbreeding territories in birds was analyzed in a migratory population of Robins (Erithacus rubecula) over-wintering in southern Spain. The behaviour of ringed territorial Robins was compared with that of Robins classified as floaters, ringed or not, during the winter of 1992- 1993 to test two non-exclusive hypotheses. First, the good feeding patch hypothesis, that territories are the most profitable feeding areas, is not supported because body condition (fat score, body mass and body condition index) and feeding rates did not differ between owners and floaters. Second, the antipredation hypothesis, that owners select protected areas with good cover to reduce mortality, does account for the maintenance of winter territories since owners spent a higher percentage of time hidden and were able to forage in lessexposed areas (i.e. closer to shelter) than floaters. Consequently, the winter territory of Robins should not be interpreted as a long-term defense of the most rewarding foraging patch. Instead, a good refuge and shelter seem to be the most important properties of territories. Thus, by attacking the intruders, owners are defending a safe foraging area close to shelter. The anti-predation benefits (i.e. a familiar and safe foraging area as well as a good roosting place) would explain per se the maintenance of winter territories in Robins and the intense competition with conspecifics and other ecologically similar species wintering in the Mediterranean.