Asymmetric territorial contests in the European robin: the role of settlement costs.

A widespread feature of territorial systems is that residents almost invariably defeat challengers. This phenomenon has been explained by the existence of value asymmetries, variations in resource-holding potential or an 'owners always win' convention. Removal-replacement experiments were performed on 75 robins, Erithacus rubeculato test these hypotheses. The settling behaviour of newcomers was also examined in order to identify energetic costs incurred during territory establishment. In winter, dominance shifted gradually from removed owners to newcomers with increasing time of newcomer residence, and there was a peak in contest duration at 4-7 days. Removals of newcomers, followed by replacement with another newcomer, confirmed that dominance was determined by the time newcomers were in residence rather than the time original owners were absent. These results support the hypothesis that asymmetries in territory value govern the outcome of contests. It is proposed that high singing rates and low foraging rates of newcomers settling boundaries with neighbours contribute to this asymmetry, skewing outcomes in favour of original owners until replacements are fully established. The key result in this study is that the time at which dominance tends to reverse (5-6 days in winter) is predicted by the time taken for newcomers to settle territory boundaries and achieve base-line foraging effort. In spring, original owners become subordinate almost immediately after removal. Reductions in settlement cost for newcomers, and increases in territory value, are proposed to accelerate dominance reversal. Age and sex effects on dominance suggest that the value asymmetry rule is modified by variations in resource-holding potential.