Dominance relations and agonistic behaviour of Tundra Swans (Cygnus columbianus columbianus) during fall and spring migration

Abstract

Social interactions and agonistic activities of Tundra Swans (Cygnus columbianus columbianus) were documented at Long Point, Ontario, to determine (i) dominance relations among social groups and (ii) the frequency and intensity of agonistic acts by swans. Families were involved in one-third as many interactions as were nonfamily groups. Nonfamily groups initiated interactions with other nonfamily groups more often than they did with family groups, but families initiated the same relative numbers of interactions with family and nonfamily groups. Further, families won nearly all conflicts with nonfamily groups, which suggests that they generally dominated nonfamily groups. Tundra Swans also showed a dominance hierarchy based largely on family or group size. To reduce conflict and energy expenditure, swans may use a simple "decision rule" during interactions: larger groups and initiators win. Two results supported this: (1) swans initiated interactions more often with smaller groups and (2) groups that initiated won interactions more often than did recipients. Swans used low-intensity agonistic acts more frequently than higher intensity ones when engaging members of other social groups, but intensity of aggression was independent of group status. Dominance based on the use of simple cues may be especially beneficial in unstable social environments at major staging areas.