Social rank and winter survival in the Willow Tit Parus montanus

Abstrakti

The dominance hierarchy and the effect of the social status on survival was studied in 18 winter flocks of the Willow Tit near Oulu northern Finland in 1986-87. The flocks were small and stable groups that usually consisted of an adult pair and apair of yearlings, which were never descendants of their older group companions . In general, males were ranked higher than the females. Within sex groups, adults dominated yearlings . The birds with long tarsi dominated individuals with shorter ones . Wing length was not correlated with social status . The size variables had no effect on rank order in group members of the same age and sex. The dominant individuals had higher survival rate than the subordinates . In juvenile birds the survival rate of high-ranking individuals was higher in both sexes. Reasons for these differences, and explanations for the existence of subordination as a strategy, are discussed. Subordinates, although having a higher mortality rate, are supposed to have even higher one if they adopt a solitary way of life. Several benefits attached to living in groups evidently outweigh the costs caused by dominant individuals .