2000 Volume 49 Issue 1 Pages 17-30,64
To examine the effects of food availability on the organization of mixed-species feeding associations in Tundra Swans (Cygnus columbianus), Common Pochards (Aythya ferina) and Northern Pintails (Anas acuta), we observed their social and feeding behavior before and after the addition of artificial food (rice grains). The feeding associations occurred when the swans paddled their legs underwater to forage for sinking food. Pochards dived under paddling swans and Pintails foraged around the feeding associations of the two species. Before the addition of food, Tundra Swans showed the largest increase in dipping frequency in the three-species associations because more sinking food was stirred up by the feeding actions of Pochards. Pochards were also found to dive more frequently and for shorter periods in the three-species associations than solitary. Pintails in the mixed-species associations obtained more food by using three feeding techniques and by shortening the distance moved while dabbling. After the addition of rice grains, association size and aggressive interactions increased. Because frequent diving by many Pochards disturbed the feeding swans, the swans in the three-species associations had the lowest dipping frequency. However, Pochards and Pintails gathered around feeding swans despite the swans' aggressiveness and they still achieved greater feeding frequencies in the three-species associations than solitary. Therefore, the functional significance of mixed-feeding associations before the addition of food was based on mutualism but, after rice grains were provided, it was based on parasitism in which the hosts were Tundra Swans and the parasites were Pochards and Pintails. The most abundant participants in the three-species associations were Pochards and Tundra Swans were mainly aggressive toward them. Adding food led to an increase in the number of Pochards, which was costly for Tundra Swans. We concluded that the organization of mixed-species associations depended on the number of Pochards joining the feeding association.